Dendrocometes paradoxus. - Journal of Cell Science...DENDROCOMETES PARADOXUS. 327 Europe it also...

DENDROCOMETES PAKADOXUS. 325

Dendrocometes paradoxus.

Part I.—Conjugation.

By

Sydney J. Hickson, M.A., D.Sc, F.R.S.,Beyer Professor of Zoology in the Owens College, Manchester

Assisted by

Mr. J. T. Wadsworth.

With Plates 17 and 18.

INTRODUCTION.

THERE is no species of the class Acinetaria that can beobtained with greater facility at all times of the year thanDendrocometes paradoxus (Stein). This is due to thefact that it is found with unfailing regularity on the gills ofour commonest fresh-water Crustacean, Grammarus pulex.

In addition to the fact that it is readily obtained, however,it presents us with the further advantage of being attachedto a soft and tolerably transparent gill, which can be easilyseen with the naked eye, and manipulated without difficultyon the slide and in the paraffin bath. The difficulties thathave usually to be overcome in preserving and staining theInfusoria whole or in sections are, in this form, largelyobviated, and it is not very difficult, after the preliminary exa-mination of the specimens attached to any one gill, to imbedand cut them into a series of sections in any plane that maybe desired. Possessing these advantages, it was obviousthat a careful examination of the changes of the nuclei

VOL. 4 5 , PART 3,—NEW SERIES. Z

326 SYDNEY J. HIOKSON.

during conjugation and gemmation might yield results ofinterest and importance.

I have had preparations of Dendrocome.tes under observa-tion for a considerable number of years, but it was not until1899 that I obtained specimens which clearly showed themitotic figures of the division of the micronuclei, and then Idetermined to investigate the matter with greater thorough-ness. I may explain that during the last two years I haveentrusted the mounting of the preparations, as well as thestaining and cutting of the sections, to Mr. J. T. Wadsworth,and that he has saved me an immense- amount of time andlabour in pointing out to me the slides that exhibited interest-ing features, and in keeping a catalogue of the preparations.The discoveries therefore that are here recorded were, in thefirst instance, made by him, and I have to acknowledge herehis unfailing assistance and perseverance in the investigation.

The structure of Dendrocometes has been investigated byBiitschli (1), Wrzesniewski (27), Plate (21), Maupas (19),Schneider (24), and Sand (23) ; but, notwithstanding then-excellent work, many points of interest and importance re-main to be illustrated and described.

Dendrocometes paradoxus is found attached to thegills of Gammarus at all times of the year, but in the summermonths there are usually fewer specimens on each gill thanin the spring and autumn. We have found that by keepingabout twelve or fourteen Gammarus in a shallow pie-dishcontaining an inch or two of water, with a little mud andwaterweed at the bottom of it, the number of Dendrocometeson the gills increases, and that in a fortnight's time a con-siderable number of pairs may usually be found in a state ofconjugation.

Dendrocometes appears to have a wide geographical dis-tribution. I have found it myself on Gammarus at Oxford,Cambridge, in the neighbourhood of Manchester, and onspecimens of Gammarus sent to me by Mr. Bolton from Bir-mingham. It was observed by Prof. Lankester on Gam-marus in the ponds at Hampstead. On the continent of

DENDROCOMETES PARADOXUS. 327

Europe it also appears to be generally distributed. I tcertainly occurs in Belgium (Sand), France (Maupas), Ger-many (Butschli, Plate, etc.), Russia (Wrz.). I t also occursin North America (Butschli). I t is probable that the Den-drocometes which occur on the gills of Grammar usp u t a n e u s (Lachmann) may be a distinct species, but, apartfrom this, Dendrocometes only occurs on the gills of Gam-m a r n s p n l e x , although exceptionally a few specimensmay be found attached to the hairs on the legs of the samehost (Sand).

D e n d r o c o m e t e s p a r a d o x u s has a planoconvex-shapedbod}7, and is provided with three, four, or five arms (PI. 17,fig. 1). The morphology of these arms is a matter of somedispute, but it is not an unreasonable view to regard them ashomologous with a bundle of Acinetarian tentacles. Theycapture, kill, and partially digest the prey in the same manneras the tentacles and suckers of other Acinetaria. The bodycontains a single large meganucleus, and a variable number,but usually three, micronuclei. There is a single large con-tractile vacuole. Reproduction is effected by an interestingprocess of internal gemmation, first described by Butschli(1). The single planoconvex bud that is formed is fre-quently called the " embryo," but I think it is more appro-priate to call it the "gemmula ." I t escapes from theparents without arms, but provided with a girdle of threebands of cilia, and swims away.

From time to time, or perhaps, under certain conditionsonly, nearly the whole substance of an individual escapes fromthe pellicullar sheath and swims away from the gill in a formwhich cannot be readily distinguished from a gemmnla.

With this brief introductoiy account of the structure andreproduction of the animal I pass on to the phenomena ofconjugation, to which I have paid special attention.

Part I.—Conjugation.

The phenomena of conjugation in Dendrocometes may bebriefly stated as follows ;

328 SYDNEY J. HICTCSON.

Two individuals in proximity on a gill of G-ammarus sendout simultaneously blunt lobe-like processes, which may becalled the conjugative processes. These meet but do notcompletely fuse, a distinct membrane delimiting the processof each individual throughout the conjugation. This mem-brane does not prevent the fusion of the meganuclei nor ofthe conjugative micronuclei in the latter stages, nor does itprevent a certain amount of mixture of the cytoplasm of theconjugating individuals.

S t a g e A (PI. 17, fig. 2).—In the initial stage one or both themeganuclei may exhibit pseudopodial processes indicating thatthey have some power of amoeboid movement. The micro-nuclei are a little but sometimes very little larger than theywere before conjugation. (In this respect, as in so manyothers, there is considerable variability. The reader willnotice that the individuals drawn in fig. 1, in which thecoujugative processes have not yet met the micronuclei, areactually larger than they are in the individuals drawn infig. 2, which are actually conjugating.)

S t a g e B (fig. 3).—The micronuclei increase considerablyin size during this stage, the chromatin being resolved intoa delicate skein. The meganuclei also increase in size,become spindle-shaped, and show an arrangement of thechromatin into roughly parallel lines. From this stageonwards until Stage K (fig. 13) is reached, the meganucleiincrease in size without showing any material change instructure ; and as the interest of the phenomena now centresin the micronuclei, further reference to the behaviour of themeganuclei will be for the present omitted.

S t a g e C (figs. 4, 5).—When the micronuclei have reachedtheir full size the chromatin collects in the form of numerousminute chromosomes in an equatorial plane. At the sametime extremely delicate, faintly staining threads (lininthreads) roughly parallel with one another, forming a tub-shaped spindle, make their appeai'ance. Neither in this norin any other stage of mitosis is there any sign of thepresence of centrosomes or similar bodies. I have found it

DBNDEOCOMBTES PAKADOXfJS. 329

impossible to count the number of chromosomes with anydegree of accuracy in this or in any other stage of division.The mode of division of the chromosomes is also verydifficult to determine, but I have seen V-shaped appearances,PI. 18, fig. 13, very similar to those figured by Prowazek (22)in Bursaria. I believe with him that these appearancespoint to longitudinal division.

The chromosomes separate into two parties, which travel tothe opposite poles of the spindle (fig. 5), where they apparentlyfuse to form a solid irregular lump of chromatin. Thespindle then elongates enormously, so that the two chromatinbodies are sometimes separated from each other by adistance equal to three fourths of the full diameter of theDendrocometes. The spindle then becomes detached fromthe chromatin and dissolves in the cytoplasm (fig. 6). Thestages in the division of the micronuclei are usually syn-chronous in the two individuals, but sometimes one set offigures is a little in advance of the other, as shown in fig. 5.

By this division six micronuclei are formed in thoseindividuals which began the process with three.

S t a g e I) (fig. 7).—One of the five micronuclei of eachindividual passes down the conjugative process to a positionvery close to the membrane, where it enlarges and againforms a mitotic figure. The other micronuclei degenerateand disappear.

The nuclei which are found close to the membrane giverise by their division to the germ nuclei, to use the termemployed by Wilson (26). The division is always in a planeparallel with the membrane (fig. 8).

S t a g e E (fig. 9).—The germ nuclei take up such a positionin contact with the membrane that each germ nucleus of anindividual is exactly opposite one of the other individual.These nuclei consist of a clear vacuole containing a singlecoarse skein of chromatin. The spindle entirely disappears.

S t a g e F.—The germ nuclei then fuse in a manner shownin PI. 17, fig. 10, and PI. 18, fig. 12, giving rise to thecleavage nuclei of the two individuals.

330 SYDNUY J. H1CKSON.

Attention may here be directed to two important points ofcomparison with ciliate Infusoria. The difference betweenthe migratory or male germ nucleus and the stationary orfemale germ nucleus is in Dendrocotnetes reduced to aminimum. It is possible that in all cases one germ nucleustraverses the membrane and the other does not, so that thedistinction remains, but the two nuclei are as nearly neutersas can be. In the second place, the fusion of the germnuclei takes place during a resting and not in a mitotic state.

According to the researches of Ma upas, Hertwig, andothers, the germ nuclei of the ciliate Infusoria fuse when inthe form of spindles or mitotic figures.

Stage Gr (PL 17, fig. 11).— One of the cleavage nucleipasses into each of the conjugating individuals and preparesto divide again by mitosis. The early stages of this divisionprobably occur very soon after the fusion of the germ nuclei,as the figures may be seen sometimes quite close to themembrane (cf. PL 18, fig. 13). This stage may be dis-tinguished from Stage D, which it somewhat resembles, bythe fact that the axes of the spindles are not parallel.

Stage H (PL 17, fig. 12).—The cleavage nucleus dividesinto two nuclei which take up a position in close proximity tothe meganucleus.

Stage J (PL 18, fig. 1).—The nuclei formed by the divisionof the cleavage nucleus again divide, and almost immediatelyone of the four becomes a little larger than the otherthree.

Stage K (PL 17, fig. 13).—The largest of the fournuclei of the last stage becomes the new meganucleus, theother three the new micronuclei. There is some evidence toshow that occasionally two of the three smaller nuclei againdivide in this stage, giving rise to a condition in which therearc six nuclei in all, as seen in the preparations from whichPL 18, fig. 8, was drawn. In some cases, too, it appearsthat two nuclei enlarge to give rise to new meganuclearstructures, as seen in PL 18, fig. 19. Variations of thiskind at these stages have added very much to the ordinary

DENDliOCOMETES PARA.DOXDS. 331

difficulties of the investigation, but the establishment of thefact that variations of such an important character do occuris, in my opinion, a result of considerable interest.

At some time during the last three stages (H; J, K) the oldmeganucleus becomes very large, and is bent on itself in theform of a loop or horseshoe. One extremity of this figurepasses into the conjugative process, and approaching thelimiting membrane traverses it and fuses with the correspond-ing extremity of the meganucleus of the other individual.The exact phase at which this meganuclear conjugation takesplace seems to vary considerably; all that can be said atpresent is that, so far as my experience goes, it usuallyoccurs between Stages J and K. The number of cases ofactual contact of meganuclei observed by Mr. Wadsworthand me is small, and this may be interpreted to mean eitherthat the period of meganuclear conjugation is very short orthat it does not always occur. Until some satisfactorymethod is invented of watching the nuclear phenomena of theconjugation of living Dendrocometes, it is impossible toprove that the meganuclear conjugation never fails. I aminclined to believe that it always occurs. Similarly I have noproof to offer of the length of time occupied by this process;but I am inclined to believe, on the circumstantial evidenceat my disposal, that it is very brief.

Soon after the meganuclei have conjugated they separateand begin to degenerate.

The usual phenomena of nuclear change during conjuga-tion in Dendrocometes may be represented by the followingdiagram, in which the circles a,bove M represent the stages inthe meganucleus, and the black dots above m, m, m, the stagesin the history of the micronuclei. Conjugation of the mega-nuclei usually occurs in Stage J, as explained above, and inStage K these bodies disintegrate.

In the following diagrams I have endeavoured to interpretcertain phenomena which appear to be variations of the moreusual stages. In Diagram B, which starts with Stage G,the important variation is that two of tho uiicronnclei formed

332 SYDNEY J. HICKSON.

by the second division of the cleavage nucleus divide again(seep. 330).

In Diagram C we have the same variation as in Diagram B,

^-o-o-o-o-o-o

0)crd

CO

o p

with the additional peculiarity that two of the nuclei giverise to new meganuclei in Stage K, and I have added inStage L the suggestion that these two new meganuclei

DENDROCOMKTES 333

fuse together to form the single meganucleus of the newindividual.

The phenomenon of conjugation in the Acinetana has been

Dicx.cjra.rn. B .

known for a great many years. It has been observed inseveral species of Acineta by Claparede and Lachmann (4),Fraiponfc, D'Udekem, Keppen (15), and others; in

D i a g r a m C

Metacineta (Claparede and Lachmann and Lieberkiihn), inPodophrya, Tokophrya, Stylocometes, and in Dendrocometes.In Dendrocometes it was observed by Wrzesniowski (27), butmore fully described by Plate (21).


As regards the external features very little can be added toPlate's description. As he observes, coujugation usually occurswhen the gill of the host is unusually crowded with the Dendro-cometes. The two conjugating individuals are in uiost casessimilar in all essential respects. Occasionally, however, a dif-ference may be observed between the two conjugates. Platestates that sometimes one of the individuals which is clearerthan the other withdraws its tentacles during the process. Sand(23) says, " Chez Dendrocometes la conjugaison a lieu souvententre un individu amaigre et un animalcule bien nourri."Mr. Wadsworth and I have frequently observed differencesbetween the conjugating individuals, and Mr. Wadsworthhas observed the retraction of the arms of certain individualsduring conjugation.

Plate observed occasionally a conjugation of three indi-viduals. This I am able to confirm, but the occurrence is sorare that no series of nuclear changes in them have beenfollowed.

As Plate denied the existence of micronuclei, he failedto see any of the stages of their division and conjugationwhich are described in this paper. In one of his figureshe shows the points of the meganuclei in the bases of thecoujugative processes, but he did not observe the fusionof these bodies. He gives a good figure to illustrate thefragmentation of the meganucleus at the close of conjugation.In Schneider's (24) figures of Dendrocometes in the act ofconjugating the meganuclei are shown to be approachingmuch more closely than in Plate's figure, and in the sameauthor's figures of the closely allied genus Stylocometes themeganuclei are actually shown to be in contact. The state-ment made by Plate and Schneider that the new meganucleusis formed by a regeneration of the fragments of the old isnot correct.

The mixing of the cytoplasm in the conjugative process isaffirmed, and I believe correctly, by Plate, Sand, and others.I have myself observed a flow of protoplasm passing back-wards and forwards through the membrane with each

DteNDROCOMETES PAtiADOXUS. 335

contraction of the vacuole for several hours in one pair, butthe period during which this occurs is limited, and duringthe greater part of the forty-eight or more hours duringwhich the pair remain in conjugation no interchange ofparticles can be seen.

An important feature of the conjugation of this genusis that the ordinary vital processes of the individuals arenot materially affected during the act. The arms remainfully extended—they catch and swallow particles of food,which are digested in the ordinary course. The contractilevacuoles continue their pulsations for some time after theprocess has commenced.

C o n j u g a t i v e Processes .—These processes are formedspecially for conjugative purposes in Dendrocometes. Indevelopment and in structure they differ from ordinary arms,and I do not consider that they are rightly considered to behomologous with arms. This view does not agree" with thatexpressed by Biitschli (2), who, on the strength of theobservations of A. Schneider on Styloconietes, regards themas rudimentary arms. In Stylocometes, according to thatauthor, an arm of each of two neighbouring individualsbecomes abnormally thick and elongated to form the conjuga-tive process, and that ib is a true arm is proved by the factthat it contains a canal. I have had no opportunity ofexamining Stylocometes, but I do not feel that Schneider'saccount of it is convincing. Until his account is confirmed,therefore, I must agree with Plate that the conjugative pro-cesses in Dendrocometes are not homologous with the arms.

Butschli (2) states that in Dendrocometes one individualsends out a process which fuses with the body-wall ofanother, but that such an occurrence is rare. I have neverseen such a phenomenon. In nearly all cases the conjugativeprocesses begin and grow simultaneously, so that they arethroughout approximately equal. In the preparation fromwhich fig. 1 was drawn, the individual to the left has adecidedly longer process than the individual on the right, butsuch a difference as this is exceptional. I t is impossible to


state what stimulus there is that causes two individualsto begin the sexual proceeding. It may be that someexcitement may be caused by the touching or brushing of thearms, but nothing of the kind has been observed. Judgingfrom the development of the conjugative processes alone,which is all that we have to guide us in the initial stages, itseems probable that the sexual stimulus affects the twoindividuals simultaneously, and that there is no differentiationof sex.

The Micronuclei.

It is an interesting fact that, notwithstanding the carefuland elaborate investigations made by several observers onthe micronuclei of the Ciliata, there is at present no satis-factory account of these structures in the Acinetaria.

That micronuclei do occur in all Acinetaria is not yet proved,but nevertheless it is extremely probable that they are asconstant a feature of the anatomy of this group of animals asthey appear to be of the Ciliata.

In Biitschli's great work on the Infusoria (p. 1873) thefollowing notes will be found on the subject. Biitschli him-self in 1867 discovered a micronucleus in a species of Sphse-rophrya. Maupas proved with certainty the existence of amicronucleus in Tokophrya limbata, Acineta tuberosa,Podophrya fixa, and Podoplirya cyclopum. He pro-bably, but not with certainty, found them also in AcinetaJolyi, whilst certain bodies which may have been micro-nuclei were seen in Ephelota gemmipara. In Toko-phrya l imbata Mobius also proved the existence of micro-nuclei.

In a recent paper on the anatomy of a new species ofEphelota, Ishikawa writes with some uncertainty about theexistence of micronuclei. The most satisfactory figures ofthe micronuclei of Acinetaria are those given by Keppen (15),but as his monograph is written in the Russian language Iam unable to read it.

The authors of the (Traite technique de Zoologie ' (5) accept

DENDKOUOMETES PARADOXUS. 337

the view that there is a micronucleus in the body of theAcinetarian.

In the recently published monograph of the Acinetaria,however, Sand (23) denies the existence of true micronuclei.He believes, however, in the existence of a body lying closeto the meganucleus, which he calls the " centrosome."

" Un corps un peu plus colorable que le noyau, creuseeexpres pour le recevoir. . . . Quand ce corps est vu aucentre du noyau, il parait entoure d'une petite zone claireformee par le cytoplasma qui le separe du noyau." Hefurther states that no Acinetarian ever contains two of thesebodies, and that it is absolutely homogeneous, both in a stateof rest and of division. He has proved the existence of thisbody in sixteen species. I have very little doubt that the" centrosome " of Sand is not a " pseudo-micronucleus," buta true micronucleus. Maupas (20) says that his series ofstages of the micronuclei of Podophrya fixa is incomplete,and he would probably himself admit that the figures he haspublished of them are not satisfactory. To account for hisunsatisfactory results in the Acinetaria, Maupas venturesupou the statement that the micronuclei of these Infusoriaare smaller than they are in the Ciliata, and stain verylightly. He adds that the study of the nuclear phenomenaof the Acinetaria is beset with so many difficulties that heconsiders it to be "une des recherches les plus peniblesqu'nn micrographe puisse entreprendre."

In Dendrocometes, however, these excuses cannot be putforward, as the micronuclei are even larger than they are inmany Ciliata, and their affinities for certain stains are ex-ceedingly powerful.

The presence of micronuclei in this genus has, neverthe-less, been denied.

Plate (21) described a number of small bodies in the cyto-plasm which, on account of their affinities for safranin, hecalled the " Tinktinkornchen," but he denied the existence of" Nebenkerne." Maupas replied to Plate's paper by statingemphatically that micronuclei do occur in Dendrocometes


and referring to Plate's own figure, pi. vi, fig. 17, as afford-ing a convincing proof of it. Neither Butschli (1), Wrzesni-owski (27), nor Schneider (24) appear to have observed them,and Sand (23) only mentions one " centrosome" as beingpresent in this genus.

The number of micronuclei in Dendrocometes varies, but,as stated above, the usual number is three. In individualsthat are neither conjugating nor preparing for gemmation itis sometimes difficult to count the micronuclei, as they arevery small, and are difficult to distinguish from other bodiesin the cytoplasm which have affinities for the stains that areused. However, a certain number of individuals withmoderately clear cytoplasm can nearly always be found, andin these it is not difficult to recognise the micronuclei in sec-tions stained by the iron-liEematoxylin and iron-brazilinmethods (13). There are sometimes two micronuclei, andsometimes four, and but rarely five. I do not believe thatthere are ever more than five or less than two.

In the state of rest the micronucleus consists of a small,irregular granule of chromatin, enclosed in a clear zone,which is invariably perfectly spherical in shape. There is noevidence of a definite membrane surrounding the clear zone.The zone is from 2—4 fx in diameter. It is very difficult toform an opinion of the nature of the substance composing theclear zone. When the nucleus is in a state of rest this zoneresists the action of all the stains I have tried, and no linesnor granules of any kind or description can be seen in it. Itis possible that it may be a mere artefact, due to the shrinkageof the chromatin during the process of preservation, but theregularity of its shape and its relation to the chromatingranule do not support this view. In my opinion it reallyrepresents the " achromatin " elements of the nucleus.

When preparing for division the micronuclei increase con-siderably in size, the solid granule of chromatin becomingconverted into a coarse skein (PI. 18, fig. 14m). Later theskein breaks up into a much finer tangle (PI. 17, fig. 3), whichgradually fills up nearly the whole of the clear zone. In


the next stage there is a differentiation of the skein into anumber of lines, which stain very faintly in iron-haemato-xylin, but a little more deeply with iron-brazilin, and a bandof deeply staining rods and granules having the usualchromatin reactions. At this stage the micronucleus isfrequently 10 /x in diameter (PI. 17, figs. 4 ,11,12) . The exactdetermination of the nature of the equatorial band of chro-matin granules is a matter of considerable difficulty. Withthe highest powers of the microscope the granules or minuterods appear to be connected together in the manner shownin PI. 18, fig. 13, but still I do not feel so far convinced ofthis connection as to deny the proposition that they areisolated chromosomes. With lower powers of the microscopethey have every appearance of being a band of rod-shapedchromosomes (PI. 17, fig. 12). Whatever the future mayreveal regarding these bodies, I think it is clear that thechromosome elements are numerous,—too numerous, in fact,to count with any degree of accuracy.

The micronucleus next becomes somewhat oval in shape(PL 18, fig. 16), and the band divides into two bands. Thefaintly staining lines, which we may call the linin threads,are aiTanged in a roughly parallel manner forming a some-what tub-shaped spindle. They do not come to a point ateach end of the figure, and there is never any dot or granulethat can suggest the presence of a centrosome. In the nextstage (PI. 18, fig. 17) the two bands of chromosomes havepassed to the extremities of the figure, and soon becomeaggregated together to form a single irregular lump. Insome cases there is a clear vacuole present at one of thepoles (the right pole in PI. 18, fig. 17), but I have seen it sorarely that I am at a loss to understand its meaning. Inthe next stage (PI. 18, fig. 18) the spindle becomes verymuch more elongated, so that the whole figure may be25—30 ft in length; the chromatin is in the form of a singlespherical lump surrounded by a clear zone at the points othe spindle. The spindle next becomes detached from thenew micronuclei (PI. 17, fig. 6), and gradually dissolves in the


cytoplasm. Throughout this mitosis the clear outline(membrana limitans ?) is never lost.

The mitosis of the micronuclei of Dendrocometes has con-siderable resemblance to the mitosis of the micronuclei ofParamcecium, as described by Hertwig (8), of Bursaria, asdescribed by Prowazek (22), and of other Ciliata.

The Meganucleus.

In specimens of Dendrocometes that are neither con-jugating nor preparing for gemmation, the meganucleus isusually a spherical body 0*02 mm. in diameter. It is not,however, constant in shape, many examples being found thatare oval or even spindle-shaped.

It is usually situated in the centre of the animal's body,but it is often more or less excentric. On examination insection with an oil immersion lens, there mny be seen adistinct mesh work of darkly staining lines which appear tosupport a series of minute rounded chromatin granules (PI. 18,fig. 14). In many cases the lines connecting these granulescannot be easily seen when the appearance is as shown inPI. 18, fig. 10. In the meshes of the darkly staining chromatinthere is a homogeneous substance, which in many iron-hEematoxylin preparations is quite colourless, but stainsfaintly yellow with brazilin.

Plate states that in the nucleus there are a number ofnucleoli: "Bei einem Thier meiner Praparate ist der ganzeKern dicht erfiillt von solchen Binuenkorperchen deren jedesvon einem hellen Hof umgeben ist." I have frequently seenin my preparations appearances similar to this describedand figured by Plate, but my interpretation of them isdifferent. In the first place I must state very emphaticallythat in my opinion there is never in the meganucleus ofDendrocometes any body or bodies which correspond with thenucleoli of Metazoan cells. The clear space round the "Binnen-korperchen " of Plate is in my opinion due entirely to therefraction of the light in passing through the preparation;and this opinion has been thoroughly tested by comparison


of whole mount preparations such as Plate studied, with thethinnest sections of the meganncleus. My opinion is thatthe substance in the meshes of the chromatin is quite homo-geneous. The periphery of the meganucleus is not in myopinion surrounded by a definite membrana limitans, notwith-standing Plate's statement that ffein Kernmembran ist deutlicherkennbar."

When the meganucleus is spherical or oval in shape andsituated at the centre of the body, or in other words, whenit is at rest, no definite membrana limitans can be seen (PI. 18fig-. 10) ; but in the elongated and dividing meganuclei thechromatin network at the periphery is so arranged that alimiting membrane seems to surround the whole nucleus-The limiting membrane is not a definite and peculiar struc-ture of the nucleus, but a temporary arrangement of thesubstance of the chromatin-bearing network at the peripheryduring the nuclear movements.

During gemmation the meganucleus undergoes a simple con-striction, and is divided into two parts, one of which is retainedby the parent, and the other by the young bud (PI. 18, fig. 14)«There is no reason to believe that these two parts are exactlyequal in size. In many preparations the part retained bythe bud is apparently smaller than that retained by theparent, but, as I have no means of measuring the capacityof these bodies, I cannot make any positive statement onthe subject. There is, however, no arrangement of thechromatin in rods or bars during this division which wouldsuggest equivalent chromatin division. Nor have I beenable to find after very careful search anything of the natureof anterior fibres, centrosomes, asters, or other charac-teristic features of karyokinesis. There can be no doubtwhatever tliat when the maganucleus divides the processis purely amitotic.

T h e F u s i o n of t h e M e g a n u c l e i d u r i n g C o n j u g a -ion.—Whatever difficulties there may be in finding an

explanation of the fact, there can be no doubt that themeganuclei do, during conjugation, meet and become

VOL. 4 5 , PART 3.—NEW SERIES. A A


continuous. The statement of this fact was made in mypreliminary communication (Hickson, 1900, 12). I am notthe first, however, to maintain that the meganuclei of theInfusoria fuse. In 1867 Stein made the following statementconcerning the Acinetaria (Stein, ' Der Organismus/ vol. ii,p. 139), 1867 :

" Die conjugation verlauft auch bei denjenigen Acineten,bei welchen sie bisher genauer studirt wurde, im Wesent-lichen auf dieselbe Weise, wie bei der gleichartigenConjugation der Vorticellen; es verschmelzen zuerst dieKorper der beiden Acineten zu einem einzigen, und dannfliessen auch deren Nuclei in einem gemeinsamen Nucleuszusammen."

Biitschli (2) gives a figure (pi. lxxiii, 96) of two attachedVorticellids in which the meganuclei are in junction, butconsiders that this is doubtfully a case of conjugation.Schneider (24), in Stylocometes, figures the junction of themeganuclei in two individuals that are conjugating, butsuggests that this also may be a case of fission. Biitschlimay have been right as regards his Vorticellids, but such amethod of fission as Schneider suggests for Stylocometes isextremely improbable. In a recent paper by Prowazek (22)a number of new and excellent figures are given of thenuclear phenomena during the conjugation of Bursaria, andit seems probable from these that in this ciliate Infusorianthere is a junction of the meganuclei before they disintegrate.Unfortunately Prowazek's description is not very clear, andhe does not attach much importance to the phenomenon.

In my preparations of Dendrocometes I have at least threecases in which the meganuclei actually touch, but a consider-able number in which they approach one another very closelyin the conjugative processes. That the junction is notmerely casual contact, but actual organic connection, isproved by the preparation which is represented in PL 18,fig. 11. Here there is no sign of any boundary between thetwo nuclei, and the chromatin granules are fixed in such amanner as to suggest very forcibly that during life they were


flowing from one side into the other. Apart from thisevidence, however, attention may be called to the fact, whichis evident not only from my own preparations (PL 18, figs.6—8), but also from Schneider's figure of Sty loco metes, thatwhen the points of the meganuclei pass down the conjugativeprocesses they converge to the same spot on the membrane.This shows, I think, that there is some force at work whichis bringing them together. Sand refers to this in Stylo-cometes when he says, "Les deux noyaux s'approchent et seplacent dans le bras dilate vis-a-vis Tun de l'autre separesparune couche de plasma/ ' But he adds, "Pouvquoi, dira-t-on,les noyaux vont-ils se placer dans le pont qui reunit les deuxStylocometes ou les deux Dendrocometes ? C'est peut-etrepour diriger les echanges et les mouvements qui ont lieu dansce pont."

The organic junction of the two meganuclei lasts a verylittle while, I believe, and it is probably followed immediatelyby their disintegration. Each meganucleus breaks up into anumber of irregular lumps, in each of which there are at firstseveral granules of unaltered chromatin. A large piece ofdarkly staining substance is frequently present in theselumps, but in many of them the central parts are simplyvacuolated (PI. 18, figs. 15 and 19).

In the next stage the cytoplasm is filled with numberlessvacuoles, granules, and lumps (PL 18, fig. 19) of endless forms,sizes, and colourable property. Tn such a preparation as thatfrom which the figure was drawn, it is almost impossible todistinguish the various remnants of the old meganuclei fromfood bodies and micronuclei.

The mode of formation of the new meganucleus at theclose of conjugation is of great importance and interest.

I must confess that in the earlier stages of this investiga-tion I had some hesitation in believing that the newmeganucleus is formed from a product of the segmentationnucleus. The descriptions, and more particularly the figures,of the nuclei of conjugation in Ciliata by Maupas (20),Hertwig (8), Hoyer (14), and others, although unanimous


are not convincing. In none of these papers are the stagesof the enlargement of the micronuclear element to form thecharacteristic meganuclear body very complete, and it seemedto me that there was just a possibility that these authoritieswere mistaken, and that the new meganucleus arises indepen-dently in the cytoplasm, or from one or more of the oldmeganuclear fragments. This hesitation in accepting theorthodox view was due to the fact, that in the earliest stagesI had then found of the formation of the new meganucleusthere was no chromatin in its centre. It was, moreover, verymuch larger than any of the other micronuclei, and the con-nection between it and a micronucleus could not be traced.

The subsequent discovery of the intermediate stages, how-ever, removed my doubts, and now I feel that it is quite anestablished fact that the new meganucleus is formed fromone of the four nuclei produced by the second division of thegerm nucleus. Plate, Schneider, and Sand, who maintainthat the new meganucleus is formed by the reconstitution ofone or more fragments of the old meganucleus, are in error,and I believe that the views expressed by Manpas andHertwig as to the origin of the new meganucleus in Ciliataare correct.

The principal stages in the formation of the new mega-nucleus are shown in PI. 18, figs. 2—5. In the first stageone of the four micronuclei (figs. 1 and 2) increases in size ina manner very similar to that in which the micronuclei swellup just before mitosis in bud formation, or in the earlierstages of conjugation. In the preparation, the enlargedmicronucleus was 5 fi in diameter, and the others 4 /n. Thisenlargement is caused by a considerable increase in the clearsubstance, and by the resolution of the chromatin into acoarse skein. In the next stage (fig. 3) the nucleus has stillfurther increased in size to about 8 /x in diameter; thechromatin has become more diffused, and does not stain sodeeply. It is probable that the change in staining powerindicates some slight change in constitution, but there isno evidence as to the nature of this change. The greater

DENDROOOMUTES PARADOXUS. 345

part of this modified chromatin is arranged in the form ofa thick ring at the periphery. There are, however, somestrands stretching across the nucleus, and a considerablenumber of rows of granules extending from the edge into thecytoplasm. I think there can be little doubt that at this stageeither the whole or the greater part of the chromatin, in itsmodified form, passes into the surrounding cytoplasm, leavingthe new meganucleus perfectly clear and homogeneous.

The elimination of chromatin from nuclei is a phenomenonof rare occurrence in animal and vegetable cells. In thematuration of the ovum of many animals a considerableamount of chromatin is ejected into the cytoplasm. Wilson(26) says, " In these cases (Asterias, Polychserus, Thalassema,Nereis) only a small fraction of the chromatin substance ispreserved to form the chromosomes, the remainder degene-rating in the cytoplasm. Some years ago I described thefragmentation of the germinal vesicle of the StylasteridA l l o p o r a (this Journal, vol. xxix) and the distribution ofits chromatin in the cytoplasm. A similar phenomenonoccurs in Distichopora (10). I have recently devoted aconsiderable amount of attention to the ovum of Alcyonium,and in this case, too, the whole of the chromatin appears tobe ejected into the cytoplasm before fertilisation takes place.In certain insects, judging from the figures given byHenking (9) and others, tlie amount of chromatin thattakes part in the formation of the first polar figure is avery small fraction of the chromatiu originally present inthe germinal vesicle (cf. Cuenot, 3A).

That the elimination of chromatin is not confined to thenuclei of egg cells is clear from the discovery of Boveri's,that in those blastomeres in the early stages of developmentof Ascaris which are destined to produce somatic cells, " aportion of the chromatin is cast out into the cytoplasm,where it degenerates, and only in the germ cells is thesum total of the chromatin r e t a ined" (quoted fromWilson, 26). In all these cases of the elimination ofchromatin from the nuclei of ova and blastomeres there

346 SYDNEY J. H1CKS0N.

appears to be no recovery in the amount of chromatin beforethe nest division occurs. In the history of the formation ofthe new meganucleus of Dendrocometes, however numerous,granules of chromatin subsequently appear at the periphery(PI. 18, figs. 4, 7, 8), and later they invade the clearercentral parts (PI. 18, figs. 5, 15), to build up thecharacteristic chromatin network of the functional mega-nucleus. The exact meaning of this elimination and recoveryof chromatin at this stage is a mystery, but taken in con-junction with the other phenomena of conjugation, it maybe regarded as a part of the general process of protoplasmicreconstitution of the organism, which is the essential featureof the sexual act.

GENERAL CONSIDERATIONS.

Maupas, in his famous work on the conjugation of theInfusoria, expressed the opinion that conjugation is essentiallyan affair of the micronuclei; and I think that the prevailiugopinion held by zoologists who have taken a special interestin this matter, is in general agreement with this view.Biitschli's opinion, as expressed in ' Infusoria' (p. 1643), isthat the meganucleus is of the nature of a somatic nucleus(Gewebekerne), which becomes gradually exhausted (allmah-lich abgenutzt wird) daring somatic life, whilst the micro-nucleus is of the nature of the sexual nuclei of Metazoa, anddoes not become exhausted by the vital processes (keinesolche Abnutzuug erfahrt). Wilson (26) expresses veryfairly the prevalent view in this sentence : " During conjuga-tion the macronucleus degenerates and disappears, and themicronucleus alone is concerned in the essen t ia l part of theprocess."

With the general proposition that the meganucleus is ofthe same essential nature as the nucleus of the somatic cellsof the Metazoa, and that the microuucleus is essentially asexual nucleus, I am in agreement; but there are seriousobjections to be raised to the further proposition, that the only

DENDROOOMETES PARADOXUS. 347

essential process in connection with conjugation is that inwhich the niicronuclei are concerned.

Plate expressed a view that during conjugation there is arecovery of some essential substance of the nucleus from thecytoplasm (see Biitschli, 2) ; but, as Biitschli rightly pointsout, it is difficult to understand upon what grounds Plate'sview is based.

Without going further into a review of the opinions ofvarious writers on conjugation, it may be sufficient to statehere the problem which is still in need of solution. Is theinterchange of molecules of the cytoplasm of the two con-jugates during conjugation an essential part of the process ?

This question cannot be answered by direct evideuce atpresent. Whatever interchange of molecules of the cytoplasmthere may be during conjugation, no method of observationhas yet been discovered by which the course of the moleculesof one individual can be traced into the body of the other.It is otherwise with the micronuclear nucleoplasm, the peculiarstructure and staining properties of which enable us to tracewith certainty the course of one micronucleus (or germ nucleus)into the body of the other. The direct evidence which wehave in the case of the micronuclear fusion is absent in thecase of the cytoplasm. It is not reasonable to conclude fromthis alone that the cytoplasm plays no part in the process ofconjugation, nor that conjugation is simply " une affaire demicronucleus." It appears to me that there is some indirectevidence, however, on this point which is worthy of consider-able attention. If the micronuclei alone were concerned inthe process, the act of conjugation need be of very shortduration. In fact, if the germ nuclei were prepared for theirtransposition a mere momentary contact would be sufficient.It might also be conceived that such a momentary conjugationwould be of advantage to the species in lessening the dis-advantageous conditions of the conjugating phase, particularlyin the free-swimming Ciliata. In all cases, however, theconjugation is a lengthy process, lasting from twelve to forty-eight hours or more. It is inconceivable that the state of

348 SYDNEY J. H10KS0N.

insensate, helpless, defenceless syzygy would remain so longif there were nothing else of essential importance done butthe interchange of germ nuclei. The fact suggests thatthere is during the process some interchange of the moleculesof the cytoplasm, and, indeed, that the interchange or mixingof the molecules is thorough, and not partial or local incharacter. If there is during conjugation an interchange ofthe molecules of the cytoplasm such as has been suggested,it is probable that some protoplasmic streaming movementwould be noticed between the two individuals. The observa-tions on the changes or movements of the cytoplasm duringthe process are, however, very limited. Maupas observed thatnumerous granules (zooamylum) appear in the cytoplasmduring the conjugation of certain Ciliata, which he supposedto be connected in some way with the active metabolism thatis going on; but I canuot find in his writings any referenceto a streaming movement taking place between the twoindividuals. But Maupas, like Biitschli and many others,1 itmust be remembered, regarded the micronuclear phenomenaas the only essential phenomena of the process, and did notexpect to find any such flow of cytoplasm.

In Dendrocometes a flow of cytoplasm between the twoconjugates does certainly take place. This was observed byPlate and is confirmed by my own observations. Sand (23,p. 10U)goes so Far as to say that conjugation is essential ly aprocess of plastogainy, and that there is not the least mixingof the nucleoplasm of the two individuals. But Sand's viewis, I believe, as far wrong in the one extreme as the older viewis in the other.

Whether a similar streaming movement of the cytoplasmbetween the conjugates cau actually be observed in thegroup of the Ciliata or not, is a question upon which I haveno evidence to offer. But whether it can or cannot beobserved under the microscope, the intimate contact of thetwo cytoplasms renders an invisible interchange of molecules

1 Del.ige and Herouard say, " Les phenomenes iuteneurs de la conjugaisonsont SUBTOUT nucleaires."

DENDROOOMKTKS PARADOXUS. 349

possible, and the "onus probandi" really rests upon thosewho maintain that two globules of protoplasm, such as these,can remain in junction for twenty-four hours without becom-ing intimately mixed.

A third point of indirect evidence bearing upon thisquestion is afforded by the behaviour of the meganucleus ofDendrocometes during conjugation. If we regard themeganucleus as a somatic nucleus—that is to say, as a nucleuswhich is functionally .connected with all the vital functionsexcept the sexual functions of the body, and as a nucleustherefore which controls or is controlled by the greater partof the cytoplasm of an animal cell such as Dendrocometes is,—then the presence of the meganuclei in the conjugativeprocesses during the interchange of the molecules of thecytoplasm is not a matter for surprise. "Pourquoi, dira-t-on,"says Sand, '' les noyaux vont-ils se placer dans le pont quireunit les deux Stylocometes et les deux Dendrocometes ?C'est peut-etre pour diriger les echanges et les mouvementsqui ont lieu dans ce pont." I am prepared, however, to gofurther than Sand, and regard the presence of the meganucleiin the conjugative processes (le pont) not only as evidenceof their relation to the interchanges taking place in thecytoplasm, but as evidence of the necessity of the inter-change of molecules of the substance of the meganucleusitself. Dui-ing conjugation there is, in my opinion, a mixingor a shuffling of the molecules of all the essential plasms ofthe body, namely, of the micro-nucleoplasm, of the mega-nucleoplasm, and of the cytoplasm.

Concerning the conjugation of the meganuclear elementstwo or three obvious objections appear. It might be urgedthat the rarity of recorded observations of the fusion of themeganuclei in the Heterokaryota, the disintegration of themeganucleus during conjugation, and the origin of the newmeganucleus from the micronuclei, are facts which prove thatthe junction of the meganuclei during conjugation in Dendro-cometes is a matter of no essential importance.

It may be pointed out that in the majority of the Ciliata

350 SYDNEY J. H1CKSON.

the meganucleus undergoes fragmentation at an earlier stagethan it does in Dendrocometes, and consequently any con-jugation that takes place between meganuclear fragmentsmight be very easily overlooked. The fact that in his mostrecent publication Prowazek figures (figs. 27 and 30) theextension of a fragment of the meganucleus of one con-jugating Stylonychia into the body of the other, supports thesuggestion that it may occur elsewhere. The second objectionis fatal to the view I am putting forward, if it is true, that themeganucleus dies when it fragments. It is, however, reallyof the nature of an assumption to say that the meganucleusdies at the close of conjugation.

Entz, Balbiani, Gruber, Maupas, Hoyer, and Prowazek areagreed in the statement that the fragments of the meganucleusare absorbed by the cytoplasm. In some species (Chilodoncucul lu lus , Colpidium colpoda, etc.) the meganucleusdoes not even fragment, it simply gradually diminishes involume and disappears. On the other hand, Biitschli (2,p. 1617) is of opinion that in Colpidium and Stylonychia thefragments of the meganucleus areejected by the anus after con-jugation. Having very carefully examined the process in Den-drocometes, and found no evidence of the rejection of any partof the meganucleus during or after conjugation, I am disposedto agree with those who believe that the old meganucleus is,as a rule, absorbed by the protoplasm. It is quite possible,however, that with the absorption of the greater part of themeganucleus there may be a rejection, in some species, of theremainder.

The expression " absorption " or " solution," as appliedto the meganucleus at this stage, is very liable to mislead.We may hold the view that the meganucleoplasm is killed,converted into some proteid food substance, and then assimi-lated by the surrounding cytoplasm, and we may use theword

DENDROOOMETES PARADOXUS. 351

we may use the word " solution" to express this meaning;but we have no evidence that it is either of these processesthat actually takes place. All the information we have isthat at a certain stage in conjugation certain structures, whichby their form and reactions to certain stains we recognise tobe meganucleoplasm, become indistinguishable from ordinaryliving cytoplasm. There is evidence of a certain change inchemical constitution, and perhaps this is only a very slightchange, and there is evidence of a certain change in con-sistency. There is really no evidence that any substanceactually dies. Theoretically, there is no inconsistency in theview that after the disappearance of the old meganucleus, itsnucleoplasm is still living in a modified form diffused throughthe cytoplasm. The new meganucleus of the Dendrocometesindividual is an enlarged and modified nucleus derived fromone of the four micronuclei which are produced by the seconddivision of the segmentation nucleus, as described above, or,to put the matter in few words, the meganucleus is derivedfrom a micronucleus. The important changes which occurduring the transition from a structure we call a micronucleusto a structure we call a meganucleus are these:—1st. A con-siderable increase in size (from 4 fi in diameter to over 12 fx indiameter in Dendrocometes). 2nd. A considerable increasein the amount of chromatin. From whence is this increasein substance derived ? I t must come either directly asformed nucleoplasm, or indirectly as food material fromwhich nucleoplasm can be formed, from the surroundingcytoplasm. The evidence as to which of these two alterna-tives is correct is not conclusive, but there is no sign of suchmetabolic activity as might be expected if the material broughtto the new meganucleus is unformed food material, and Gon-sequently it is very probable that the increase in size is dueto formed nucleoplasm transfused from the cytoplasm to thenew meganucleus. If this is the case, then the phenomenonof the conjugation of the meganuclei receives an explanation.

This view appears to me to receive considerable supportfrom the observation made by Biitschli that the posterior


fragment of the meganucleus of Euplo tes charon does notdie, but fuses with the new meganucleus. A similar observa-tion was made by Maupas on Euplotes patel la.

The investigation of the conjugation of Dendrocornetesdescribed in this paper throws no new light on the importantquestion of the initial stimulus to syzygy. It is well knownthat Maupas was able to induce conjugation in severalspecies of Ciliata by a judicious withdrawal of food materialafter a certain number of binary fissions; that he was ofopinion that in natural conditions it is the exhaustion of thefood supply which affords the main stimulus to the epi-demics of conjugation. The views of Maupas have recentlyreceived some support from the experiments of Prowazek(22), who was able to induce conjugation by hunger inSty lonychia pus tu la ta . On the other hand, Joukowsky(17) failed to induce conjugation by hunger in Pleurotrichaafter experimenting for eight months and reaching the four-hundred-aud-fifty-eighth generation.

I tried the experiment several times of isolating a numberof Gammarus bearing the Dendrocometes in filtered waterfor six days or a week, and obtained in some cases sufficientevidence that the Acinetarians were affected by hunger; butthere were on an average neither more nor less pairs inconjugation than in the Dendrocometes of the controlexperiment. Starvation cannot be extended for more thau aweek in this case, as the hosts soon die in the filtered water,and their macerating bodies afford ample food again for theepizoites.

Dendrocometes itself is peculiar among Infusoria in thatit appears to be capable of feeding all through the processof conjugation. Mr. Wadsworth and I have observed thearms of conjugates catch food and pass it down into the bodyprotoplasm. Judging from the food granules as seen insections, the onset and progress of conjugation appear quiteindifferent to the condition of hunger or satiety.

The following notes will illustrate this point:(The letter F in the third and fourth columns signifies that

DENDUOOOMETES PA1UDOXUS. 358

the conjugate contains food vacuoles, andcytoplasm is clear or moderately

Number ofSlide.146

83,8410

12912712612212125

127120119

Stage ofConjugation.

BBCCCBEEJJJJ

clear.)Conjugate

A.F.F.F.S.S.

s.F.F.S.S.F.F.

the letter S that its

ConjugateB.F.S.

s.s.s.s.s.F.

s.s.s.F.

The G-eneral Morphology of the He te rokaryo teBody.—The investigations of Maupas (20), Butschli (2)and Keppen (15), notwithstanding the writings of Plate(21), and more recently of Sand (23), have placed beyondall reasonable doubt the zoological affinity of the classesAcinetaria and Ciliata. In these two classes alone there aretwo kinds of nuclei in each independent organism. In allother Protozoa, with the exception perhaps of a few formslike Pelomyxa, in which there are only scattered granulesof chromatin, there is only one kind of nucleus.1 Thisfundamental distinction of the Ciliata and Acinetaria justifiesus in placing them together in a subdivision of the Proto-zoa, which may be called the Heterokaryota (Hickson, 11).

There may be some difficulty in giving an absolute defini-tion of what is a nucleus. It will be agreed, however, thatevery structure in a protoplasmic mass that contains

1 Caenot (3) has recently discovered that in a Gregarine belonging to thegenus Diplocystis, which is parasitic in the common cricket, the two forms ofnuclei occur. The micronucleus, however, does not become visible until theonset of sporulation, but it then divides by a vnitotic process to give rise tothe nuclei of the spores, while the meganucleus disappears.


chromatin and that divides by mitosis is a nucleus. It isfrequently very difficult, however, to distinguish true nuclearchromatin from substances in the cytoplasm that are notchromatin, and there are many examples of nuclei known toscience that do not divide by mitosis. It may be taken,however, as a further axiom of histology that every structureoriginating as a daughter nucleus by mitosis of a pre-existingnucleus is itself a nucleus.

Both the meganucleus and the micronucleus of the Hetero-karyote body, therefore, are true nuclei; the former on theground that it originates from the nucleus formed by themitotic division of a micronucleus, notwithstanding the factthat it always divides amitotically,1 and the latter on theground that it divides by mitosis. These two nuclei, how-ever, differ from each other in several important particulars.The meganucleus is very much larger in bulk during thesomatic life of the individual than the micronucleus. Infission or gemmation it divides amitotically. It does notdivide during conjugation, but during or at the close of thisprocess it ceases to exist as a definite entity.

The micronucleus, on the other hand, is very much smallerthan the meganucleus during somatic life. In fission andgemmation it divides by mitosis. It does divide, again bymitosis, during conjugation, and one of the products of itsdivision gives rise to the germ-nuclei. It is not necessaiy todiscuss further in this place the relations of these two nuclei.The reasons set forth by Biitschli with masterly ability in hisgreat work on the Infusoria, for considering the meganucleusto be the " somatic" nucleus, and the micronucleus as the"sexual" nucleus, are sufficient for my purpose. If, how-ever, we accept the view that in the body of the Heterokaryotethere is one (or occasionally more than one) somatic nucleusand one or more than one sexual nuclei, we are led to thefurther inquiry whether there is also a distinction betweenthe somatic cytoplasm and the sexual cytoplasm.

1 Apparent exceptions to this rule are afforded by the meganuclei ofOpalina and Kentrochona.


There is no evidence of a positive character to show thatthis is the case, but the absence of any visible boundary linebetween the sexual cytoplasm and the surrounding somaticcytoplasm is not a definite proof that the distinction does notoccur. Many instances could be quoted, both from animaland vegetable tissues, in which each nucleus of a plasmodiumhas its own sphere of influence in the surrounding protoplasm,even when no cell boundaries can be distinguished. It is,indeed, contrary to our general knowledge and usual concep-tions of cell structures that any nucleus should be entirelyindependent of the cj'toplasm that immediately surrounds it,just as it is that any nucleus should exist entirely free fromany cell protoplasm.

There is one feature of the sexual cells of the Metazoawhich at this point in the argument I should like to callattention to. When ova and spermatozoa are ripe, that is tosay, when they are ready to perform the only function theypossess, they ai*e entirely free from surrounding cell structures.There is no reason to believe that in any case I can call tomind the individual ovum or spermatozoon is in protoplasmiccontinuity or even contact with other cells. There are noother cells of the animal body, except the white blood-corpuscles, of which the same statement can be made, and itis a feature of some interest and importance that in theMetazoa these cells are in their mature condition independententities. Now in the Heterokaryota the sexual cytoplasmmust be in contact with, and in all probability is in continuitywith, the somatic cytoplasm at the time of maturity, andeven after the fertilisation has been effected. In this respectthen, there is an essential difference between the Metazoa andthe Heterokaryota. In the Metazoa a conjugation of thesomatic cells and of the somatic nuclei could have no possibleeffect upon the sexual cells, either before or after fertilisation.In the Heterokaryota, on the other hand, whatever effect theconjugation of the meganuclei and the somatic cytoplasmmay have, it must be felt by the sexual nuclei and the sexualcytoplasm with which thev are in contact. This consideration


throws some light on the phenomenon of the conjugation ofthe raeganuclei in the Infusoria, a phenomenon which has noparallel in the Metazoa.

In the recent discovery of the phenomenon called "Xenia"by the botanists in plants, we find a parallel although notstrictly homologous case. The ripe ovum of the angiospermis not an isolated cell. Its germinal cytoplasm is continuouswith the general cytoplasm of the embryo sac, in just thesame way as, according to my views, the germinal cytoplasmof a Dendrocometes is in continuity with the somatic cyto-plasm. It is quite possible, therefore, that anything whichinfluences the polar nuclei or the general cytoplasm of theembryo sac would influence also the ovum (oosphere) beforeor after fertilisation is effected. Nawaschin and Guignard(7) have shown that in Lilium and some other Angiospermsthe second nucleus of the pollen grain does pass down thetube, and conjugates with one of the polar nuclei to form themother nucleus of the endosperm nuclei. The second nucleusof the pollen grain and the polar nuclei of the embryo maybe compared with the meganuclei of the Heterokaryote body.Like these nuclei they conjugate at the time of the true sexualconjugation of the germinal nuclei, and, moreover, they donot by subsequent division give rise to the nuclei of the newindividual. It is true that there are important differencesbetween the two cases. In the plant the conjugation of thesenuclei is not temporary as it is in Dendrocometes, butpermanent, and the product of the conjugation gives rise to aconsiderable progeny of well-defined nuclei in the endospermbefore their history is closed. But such differences as theseare not surprising in organisms so widely separated as theInfusorian and the Angiosperm plant. Detailed comparisonof the two phenomena would probably not be profitable, andmight, indeed, be misleading. All that the comparison cando for us at present is to confirm the impression that thetemporary fusion of the meganuclei of Dendrocometes thathas just been described is an important and essential part ofthe process of conjugation, and not an exceptional or accidental


juxtaposition of the nuclei in the individual cases examined.I t may also lead to the discovery of other cases of theconjugation of meganuclei in the Acinetaria and in theCiliata.

As a general result of these considerations, it seems to methat we must either abandon the use of the expression" unicellular organisms " in our definition of the Protozoa,or else very largely extend the meaning of the term " cell."In the recent text-books published by Sedgwick and byShipley and Macbride, the former course is adopted; butLang, in his l Lehrbuch der vergleichenden Anatomie/ 2ndedition, 1901, says, i: Die einfachsten Organistnen, dieeinfachsten Thiere (Protozoa), und die einfachsten Pflanzeu(Protophyta)sindweiter uichts als selbstandigund unabhangiglebende Zellen."

The body of a Paramcecium or of a Dendrocometes is nomore a single independent cell than is the embryo sac ofan Angiosperm plant.

If we are prepared to extend the use of the term cellso as to include all structures that are bounded by anundivided cell wall or cell boundary, then the expression" unicellular " may still be applied to the Protozoa; but, inmy opinion, the inconvenience of such a course would farexceed the advantages it might present.

L I S T OF THE PRINCIPAL PAPEES REFERRED TO IN THIS

MEMOIR.

1. O.BUTSCHLI.—" Ueber den Dendrocometes paiadoxus," etc., 'Zeits. f. w.Zool.,' vol. xxviii.,1877, p. 49.

2. 0. BUTSCHLI.—" Infusoria" in Bronn's * Thierreich,' 18S9.

3. L. CUENOT.—' Evolution des Giegarines celomiquc du Grillon domcb-tique," 'C. 11.,' cxxv, p. 52.

3a. L. CUENOT.—' L'JEpuration nucleaire au debut de l'Ontogeuese,' t. c.,p. 190.

4. E. CLAPAREDE et J. LACHMANN.—'Eludes sur les Infusoires,' Geneva,1857-1860.

VOL. 45 , PART 3. — NEW SERIES. BB


5 YVES DELAGE et E. HEROUAHD.—'Traite de Zoologie conciete,' tome

i, Paris, 1896.

6. Y. DELAGE.—" Embryons sans noyau raaternel," ' C. R.,' cxxvii, p. 52S.

7. L. GUIGNARD.—" Sur les Antherozoides et la double Copulation sexuellechez les Vege*taux angiospermes," ' C. R.,' cxxviii, p. 864.

8. R. HERTWIG.—"Ueber die Konjugation der Infusorien," 'Abh. derbayr. Akad. der Wiss.,' Cl. II, Bd. xvii.

9. 0. HENKING.—" Untersuchungen \iber die ersten Entwickelungs-vorgange in der Eiern der Insekten," ' Zeils. f. w. Zool.,' liv, 1.

10. S. J. HICKSON.—" The Early Stages in the Development of Distichopora,with a short essay on the Fragmentation of the Nucleus," ' Q. J.Micr. Sci.,' 1893, vol. xxxv, 1.

11. S. J. HICKSON.—" The Reproduction and Life History of the Protozoa,"'Trans. Manch. Microscop. Soc.,' 1900.

12. S. J. HICKSON.—" The Nuclei of Dendrocometes," ' Reports of theBritish Assoc.,' 1900.

13. S. J. HICKSON.—" Staining with Brazilin," ' Q. J. Micr. Sci.,' 1901,vol. xliv, 3.

14. H. HOYER.—" Ueber des Verhalten der Kerne bei der Conjugation desInfusors Colpidium Colpoda," ' Arch. mikr. Anat.,' 54.

15. — KEPPEN.—"Remarks on the Infusoria Tentaculifera," 'Mem. Soc.Nat.,' Odessa, 1888, xiii, 2 (in the Russian language).

16. C. ISHIKAWA.—" Ueber eine in Misaki vorkommende Art von Ephelotaund iiber ihre Sporenbildung," 'Japan Coll. Sci. Imp. Univ.,'vol. x,p. 119 (1896-8).

17. D. JOUKOWSKY.—" Beit I age zur Frage nach den Bedeutungen derVermehrung und des Eintritts des Conjugation bei deu Ciliaten,"* Verh. naturh. med. Ver. Heidelberg,' n. F., vi.

18. E. MAUPAS.—"Sur la Podophrya fixa," 'Arch. Zool. expe*r.,' vol. v.1876, p. 401.

10. E. MAUPAS.—"Contribution a l'etude des Acinetiens," 'Arch. Zool.exper.,' vol. ix, 1881, p. 299.

20. E. MAUPAS.—"La Rajeunissemeut karyogamique chez les Cilies,"' Arch. Zool. exper.,' II serie, vol. vii, 1889.

21. L.PLATE.—"Untersuchungen einiger an den Kiemenblattern des Gam-marus pulex, lebenden Ektoparasken," 'Zeits. f. w. Zool.,' vol. xliii,p. 175,1886.

22. S. PROAVAZEK.—" Protozoenstudien," 'Arb. aus Zool. Tnst.it. Wien,'torn, xi, 3, 1899.


23. It. SAND.—' Etude monographique sur le groupe des Infusoires tenta-culiferes,' Brussels, 1901.

24. A. SCHNEIDER.—' Fragments sur ]es Infusoires. Tablettes zoologiques,'vol. i.

25. H. J. WEBBER.—" Xenia," ' U.S. Dept.of Agriculture,' Bulletin 22.

26. E. B. WILSON.—'The Cell in Development and Inheritance/2nd edition,1900.

27. A. WBZESNIOWSKI.— " Beitrage zur Naturgeschichte der Infusorien,"Z. f. w. Z., vol. xxix, 1877, p. 255.

EXPLANATION OP PLATES 17 & 18,

Illustrating Mr. Sydney J. Hickson's paper on "Dendro-cometes paradox us."

PLATE 17.The figures in this plate, with the exception of 1, 2, and 5, are constructed

from a series of drawings of the actual sections of the Dcndrocometes. Themicronuclei do not all occur in the same plane as represented, and it is veryrarely that the whole of the meganucleus can be seen in one section. Thestructures have been repiesented as nearly as possible in their true relativepositions. Throughout, M. refers to the meganucleus ; m., the micronuclei; A.,the arms; P., the conjugating process; L. M., limiting membrane. The numberat the end of the description of each figure refers to the permanent preparationfrom which the figure was drawn. Tliese prepaiations aie preserved in theZoological Laboratoiy at the Owens College, and may be inspected by qualifiedzoologists.

FIG. 1.—Two individuals of Dendrocometes about to conjugate. Each oneis protruding a conjugating process (P. P.) and these ultimately meet. In tliesetwo individuals the arms are of approximately the same size and degree ofbranching. Whole mount, No. 14.

FIG. 2.—Two individuals which have just joined together in conjugation(Stage A). One of them (to the right) has one short arm and two veryrudimentary arms (A. 2, A. 3). The other has one large branched arm and twoshorter simple arms (see p. 328). The microuuclei are very small, and themeganuclei have undergone very little change (see p. 331). Whole mount,No. 29.


FIG. 3. Stage B.—The meganuclei have become spindle-shaped and arcslightly enlarged. The micronuclei (three in each individual) are considerablyenlarged, the chromatin forming a loose meshwork. Section No. 139.

In this and the following figures details of the arms are omitted.

FIG. 4. Stage B (later).—The micronuclei now show chromosomes arrangedin equatorial planes and linin fibrils running through them to the poles. Inthis preparation only two micronuclei can be seen in each individual.Sections No. 129.

FIG. 5. Stage B (close).—The micronuclei are in later stages of theirmitosis. In the individual on ihe right the chromosomes of the micronucleihave separated into two parties travelling toward the poles. In the individualson the left the chromosomes have reached the poles and fused into a compactmass. Whole mount, No. 14.

FIG. 6. Stage C.—There are now six micronuclei in each individual, and theundissolved remnants of some of the spindles (Sp.) may be seen in the cytoplasm.The megauuelei have been omitted from the drawings to render the positionsof the nuclei clear. Section No. 139.

FIG. 7. Stage D.—Five of the micronuclei in each individual are now under-going degeneration, but one in each (m.c.) travels down the conjugativeprocess and approaches the membrane of separation. The meganucleus isomitted in the figure from the individual on the right. Sections No. 126.

FIG. 8. Stage D.—The conjugative micronuclei (me, me.) are now dividingby mitosis in the conjugative process of each individual. Sections No. 134.

FIG. 9. Stage E.—Tlie conjugative micronuclei have now divided into twoseparate nuclei.—the " germ nuclei." Sections No. 138.

FIG. 10. Stage F.—The germ nuclei of the two individuals have now fusedor are fusing (upper one). Sections No. 140.

FIG. 11. Stage G.—The cleavage nuclei (S.m.) formed by the fusion of thehalves of the conjugative nucleus in the last stage now travel towards thecentie of each individual and again show mitosis. During the precedingstages the meganuclei have been gradually enlarging and have now reached aconsiderable size. Degenerate remnants of the other micronuclei may still beseen in the cytoplasm. Sections No. 141.

FIG. 12. Stage H.—The segmentation nuclei having divided once moreshow mitosis. Sections No. 107.

For illustrations of Stage J see Plate 17, figs. 1, 7, 8.

FIG. 13. Stage K.—The segmentation nuclei have now divided into fournuclei in each individual, tlnee of which become reduced in size and thechromatin concentrated into a single granule, and one in each becomesenlaiged to fonn the new meganucleus (n. M.). The old meganucleus isbeginning to disintegrate. Sections No. 25.

DENDROCOMETES PAKADOXUS. 361

PLATE 18.

1. Section through one of a pair of conjugates alter the second divisionof the segmentation nuclei (Stage J), showing one nucleus (n. M.) slightlylarger (5 /i) than the other three (4 ft). This larger nucleus becomes the newmeganucleus. No. 122.

2. The new meganucleus from the last figure enlarged to show that thecliromatin is at this stage in the form of a coarse skein lying in the centreof a clear space.

3. The new meganucleus at a later stage. It is now about 8 n in diameter.A considerable quantity of the cliromatin has now collected at the periphery,and some of it appears to be escaping into the cytoplasm. No. 145.

4. New meganucleus at a still later stage, 10 /x in diameter. Darklystaining granules of cliromatin are now seen at the periphery, one or twowithin the peiiphery, but the central parts stain very faintly indeed. Series131.

5. New meganucleus at a still later stage, 12 /x in diameter, containingnumerous evenly distributed granules of cliromatin. Slide 131.

6. Section through a pair of conjugates showing the approach of the oldmeganuclei (M.) to each other at (he bar of junction. No. 52. (The micro-nuclei were not clearly stained in this preparation, and are consequentlyentirely omitted from the drawing.)

7. Section through a pair of conjugates (Stage J), showing one meganucleusat the limiting membrane, the other pointing towards it but not reaching it.There is one new meganucleus and three micronuclei represented in each. (Inthe preparation, owing to an unfortunate tear, only one micronucleus can beactually seen in the lower conjugate.) No. 141.

8. Section through a pair of conjugates (Stage J), showing the approach ofthe old meganuclei to each other at the limiting membrane. In each ofthese there is one new meganucleus and five micronuclei (an exceptionalcondition). In the cytoplasm of the lower conjugate there may be seen threegranules of cliromatin (?). These may be the remnants of the polar nuclei.Slide 131.

9. A small portion of the old meganucleus of one of the conjugales ofthe last preparation more highly magnified, showing the cliromatin arrangedin irregular parallel lines with thickened nodes and lumps.

10. Section through a resting meganucleus, stained by iron-hsematoxylin.No membrana limitans can be seen. No. 255.

11. Section through the conjugative processes of a pair of Dendrocomeles,showing the organic fusion of the two meganuclei in Stage J. No. 98 S.

12. Section through the conjugative processes of a pair of Dendrocometes

362 SIDNEY J. HIOKSON.

(Stage F), showing the fusion of the germ nuclei (G. m.). On the left the twonuclei have not completely joined; the chromatin is in the form of a coarseskeiu with thickened nodes. On the right the pair have fused, and thechromatin has assumed an irregular asterid form. No. 140.

13. Section through the conjugative processes after the fusion of the germnuclei to form the segmentation nuclei (S. m.). The segmentation nucleibecome mitotic soon after their formation, but the axes of the figures are notparallel with the membrane nor with one another. In this case the upper figureis seen in longitudinal section and the lower in transverse section. No. 117.

14. Section through a Dendror.ometes at an early stage in the forma-tion of a gemmula to show the normal mode of division of the meganucleus.B. B. Bands of concentric modified cytoplasm which form the peritrichousbands of cilia of the gemmula. No. 34 s.

15. Section through one of a pair of conjugates in Stage K, showing thefragments of the old meganucleus and the new meganucleus. m., One of themicronuclei? No. 92.

16—18. Three stages in the division of the micronuclei. 16. Imme-diately after the division of the chromosomes. 17. The chromosomes separatedto the poles of the figure. 18. The chromosomes collected into agianule ofchromatin at each of the poles, and the achromatin in the form of an elongatedspindle. 16 and 17, No. 129. J8, composition diawingfrom several prepara-tions.

19. Section through a Dendrocometes at the close of conjugation. Itshows the rare condition of two new meganuclei. The old meganucleus hasalmost completely disintegiated. No. 74 E.

NOTE.—In Stage K, when the old meganucleus has fragmented, it isextremely difficult to distinguish the micronuclei from fragments of the oldnieganucleus. I have theiefore made no attempt to reconstruct Fig. 15 andFig. 19, so as to show all the micronuclei in their correct lelative positions.These two figui'es were drawn with the assistance of the camera lucida fromone section only of each series of sections.

Date post:	29-Jan-2021
Category:	Documents
Upload:	others
View:	3 times
Download:	0 times

Dendrocometes paradoxus. - Journal of Cell Science...DENDROCOMETES PARADOXUS. 327 Europe it also...

Documents