194 J. CHU A Genealogical Analysis of Fission in Stylaria fossularis. By J. CHU (Department of Biology, National University of Chekiang, Meitan, Kweichow, China). [Communicated by THE SCIENTIFIC DIRECTOR-Received FebruarJ; 9, 1945.1 (With 8 figures in the text.) In a previous communication (Chu and Pai, in press), we have reported a relationship between natural fission and the rate of regeneration in Stylaria fossularis. It was found that the region in which fission occurs most frequently coincides with that in which the rates of regeneration are equal for the anterior and posterior fragments. In a wild population the level of natural fission ranges from trunk segment 8 to 25, but fission occurs most frequently at segment 18, from which segment, forwards and backwards, the frequency decreases. A satisfactory explanation of these results cannot as yet be given, and the problem requires further investigation. The work reported here is a genealogical analysis of the fission process, with special reference to the lineage of the worms and the genealogical variability of fission levels. A statistical analysis of fission frequencies and a direct comparison between the natural fission frequency and the genealogically analysed fission frequency are included. The writer wishes to express his grateful acknowledgment to Prof. S. Pai for valuable suggestions and criticism in the course of this work. Observation and Analysis, Six worms, with fission zones at segments 16, 13, 14, 16, 18 and 21 respec- tively, were arbitrarily chosen as mother worms and cultured individually. After a worm had divided, the new individuals were separated and designated worms of the first generation. The repetition of the fission process in these worms gave rise to worms of the second generation, and so on. Populations derived from these mother worms were grouped as families I, 11, . . . and VI. respectively. The worms were cultured by the method already described (Chu and Pai, loc. cit.). Since fission takes place very rapidly, daily examina- tion was necessary ; the position of the fission zone of the worms in each generation was recorded for analysis. The genealogical relationship among worms within a given population may be best illustrated by an example. Fig. 1, which was drawn to represent some early generations of family 1, can serve as such. The original worm produced, by fission at trunk segment 16, two worms of the first generation, referred to as IA and Ix respectively, distinguishing the worm originating from the anterior and that from the posterior end of the mother worm. A few days later, these two worms divided at the 15th and 18th trunk segment respectively giving rise to worms of the second generation. Succeeding generations arose in a similar way in due course. The external features of the worms thus obtained reveal the existence of three distinct worm types. For convenience’ sake, these may tentatively be termed “ primary ”, (( anterior ” and ‘( posterior ” worms. A primary worm is one which has arisen, genealogically speaking, from a single segment of the mother worm through the successive formation of new segments. An anterior worm develops from that part of the primary worm in front of its fission zone, and a posterior worm from the part behind it. These worm types are schematically represented in fig. 3, which shows not only their derivation, but their morphological characters. The primary worw
Transcript
194 J. CHU
A Genealogical Analysis of Fission in Stylaria fossularis. By J. CHU (Department of Biology, National University of Chekiang, Meitan, Kweichow, China).
[Communicated by THE SCIENTIFIC DIRECTOR-Received FebruarJ; 9, 1945.1
(With 8 figures in the text.)
In a previous communication (Chu and Pai, in press), we have reported a relationship between natural fission and the rate of regeneration in Stylaria fossularis. It was found that the region in which fission occurs most frequently coincides with that in which the rates of regeneration are equal for the anterior and posterior fragments. In a wild population the level of natural fission ranges from trunk segment 8 to 25, but fission occurs most frequently a t segment 18, from which segment, forwards and backwards, the frequency decreases. A satisfactory explanation of these results cannot as yet be given, and the problem requires further investigation. The work reported here is a genealogical analysis of the fission process, with special reference to the lineage of the worms and the genealogical variability of fission levels. A statistical analysis of fission frequencies and a direct comparison between the natural fission frequency and the genealogically analysed fission frequency are included.
The writer wishes to express his grateful acknowledgment to Prof. S. Pai for valuable suggestions and criticism in the course of this work.
Observation and Analysis, Six worms, with fission zones at segments 16, 13, 14, 16, 18 and 21 respec-
tively, were arbitrarily chosen as mother worms and cultured individually. After a worm had divided, the new individuals were separated and designated worms of the first generation. The repetition of the fission process in these worms gave rise to worms of the second generation, and so on. Populations derived from these mother worms were grouped as families I, 11, . . . and VI. respectively. The worms were cultured by the method already described (Chu and Pai, loc. cit . ) . Since fission takes place very rapidly, daily examina- tion was necessary ; the position of the fission zone of the worms in each generation was recorded for analysis.
The genealogical relationship among worms within a given population may be best illustrated by an example. Fig. 1, which was drawn to represent some early generations of family 1, can serve as such. The original worm produced, by fission at trunk segment 16, two worms of the first generation, referred to as IA and Ix respectively, distinguishing the worm originating from the anterior and that from the posterior end of the mother worm. A few days later, these two worms divided at the 15th and 18th trunk segment respectively giving rise to worms of the second generation. Succeeding generations arose in a similar way in due course. The external features of the worms thus obtained reveal the existence of three distinct worm types. For convenience’ sake, these may tentatively be termed “ primary ”, (( anterior ” and ‘( posterior ” worms. A primary worm is one which has arisen, genealogically speaking, from a single segment of the mother worm through the successive formation of new segments. An anterior worm develops from that part of the primary worm in front of its fission zone, and a posterior worm from the part behind it. These worm types are schematically represented in fig. 3, which shows not only their derivation, but their morphological characters. The primary worw
FISSION IN STYLARIA FOSSlJLARIS 195
is generally furnished with a single, anterior pair of long sets , pertaining to the original maternal segment, a.nd many pairs of shorter s e t s which have been formed anew. The anterior type is characterized by the possession of relatively large anterior segments. The posterior segments, rather smaller in size, and light yellowish brown in colour, are the derivatives of the trunk
Figure 1.
A family tree of the fir-t four generations from Family I, showing tho gonorel features of worm lineage. M.W., mother worm ; Z, levol of fission indicated by the number of trunk eogmente anterior to the fission zone ; I, 11, I11 and TV, generations ; A, antorior worm type ; Pr, primary worm type ; P, postorior worm typo ; Ix, un- known worm type determined by tho type of mother worm.
segment immedia.tely preceding the fission zone of the original primary worm. Finally, the posterior type is characterized by the uniform size of the body segments and its more slender and delicate appearance as compared with the other two types. All the above distinctions are, however, temporary dis- tinctions observable only during a certain restricted period after fission.
In accordance with this classification, the symbols Pr, A and P in fig. 1 denote the primary, anterior and posterior worm respectively ; the prefixes I to IV indicate the successive generations, while the numbers 1 ,2 . . . differen- tiate members of the same family within each generation. For iristance, in fig. I ? 11, Pr, represents a primary worm of the second generation, developed froin a single segment, i. e . the 16th trunk segment of the mother worm ; but SO far as lineage is concerned, this worm is directly derived from the worm IA. Similarly II1,Pr is a primary worm of the third generation, originat,ing from the 15th trunk segment of the worm IA and having the worm 11, A as its imme- diate ancestor. It is not known to what type the original mother worm belonged. If i t were an anterior worm, the unknown worm type (Ix) in fig. 1 would be a primary type. If, on the other hand, it were a primary or a posterior worm, then this same worm (Ix) would be a posterior type. An interesting regularity is revealed by the diagram : in a primary or posterior worm fission always leads to the production of an anterior and a posterior (laughter worm, while in an anterior worm fission gives rise to an anterior and a primary worm. Because of this difference, the expected ratio of the members of each type in a given population will be three (anterior) : two (posterior) : one (primary).
Despite the temporary nature of the morphological differences t,hat have been enumerated, the different worm types continue to exhibit certain pecu- liarities. In the first place, the posterior worm is characterized by a higher mort.ality, and this influences the ratio of the total number of worms of each type within a given generation. From fig. 2 it can be seen that this ratio should be 2 (anterior) : 1 (--rimary) : 1 (posterior) for the second generation,
13*
as wdl ;IS for any succwding generation. Now, supposing that one of the posterior worms of the t.tiird geiierat.ion were missing, the relative ratio in the next, generation would cha.nge from 8 : 4 : 4 to 7 : 4 : 3 . Another feature peculiar to the posterior worm is its relatively low fission rate as compared wit.11 that. of anterior antl primary worms (Table I). This factor can also
A" SP A" 'jr *' .~ A' pr A P A pr A' " P (PA A'" 'br
G~~ n A : pr: P = ~ + I '8 a: A : P r : P=4 : 2 : 2
1y A . P r : P = 8 : 4 4 A schematic: family treo of f'tni? gonorations showing tl ie ratio hrtween primary, anterior
R I L ~ posterior norm types.
influence t,he ratio within a given generation, though to a lesser degree. The actual ratio of the differoiit, worm types is given for families I and I11 in Table 11. Out of a t&al of 483 wornis examined a t the end of the 14t.h generation the observtd ratio was approsiniat.ely 6 : :I : 2 , a finding which diff'ers from both 2 : 1 : 1 and 3 : 2 : 1 . The author is inclined to believe that this deviation is a result of tlie pe(w1krities of thtx post,erior worm dcscribed here.
TABLE I.-Fission rate in days of the three worm types. \VOl~IYl Type. .i\ntc.rior \Voriii. I'rimriry Worm. Posterior Worm.
~- __ .___ - 9 . 7 12.6 Vissioii ratv in days 7.R
TABLE 11.-Ratio of numbers of different worm types in Pamilies I antl TIT counted a t t,he end of the 14th gcneration.
9 further point of considerable interest is the shifting of the fission zone in anterior wornis. Referring t.o fig. 3 or fig. 4*, it is seen that the fission level a t the 18th trunk segment shifted to the 17M and 16th segments respectively in the next two succeeding generations (worm IA and 1 1 , ~ in fig. 3). This regular forward shift of the fission zone by one segment a t each division con- tinues, generation by generation, until a particular limiting level is reached. 1 n I'riafinn Inngisrfn, Henipelniann (1W3), reported that the limiting level is the 12th segment. Tn the present material, however, it is not located at one definite segment but is situated at various levels in different anterior worms. When the fission process reaches the limiting lcvel, the fission zone ceases to
* I<tvcntly, Van Clcave (1!337) has described in P~i.st.inr~ /ojt(/iseto an orderly distribution of m o r v than one fission zone in R sinplr worin. This s ( ~ i i i t \ cllaractwistic. featlire is fount1 iii tlic pi,rsont, rriitterial. Fig. 4 ropresents an anterior wnrni showing foul, c-oiinrc.~~tivp fission zorws.
hTHSI0S 13 STYLARI.4 h’OSSUL9KIS 1!,7
move forward, though new segments are formed a t its post,t.rior cwtl in the usual manner. A new fission zone appears among the new segments, when ii sufficient. number of these has been added. This jump backward of the “ new fission zone )’ does not bear any definite relation to the anterior limiting level in a given anterior worm : the difference in segment number between t,hese t.wo levels varies from one to five-a vrtlur slightly larger than that, observed by Hempel- mann for I’ristinu longiseta (one t.o thrw). These fission characterist.ics of
Figure 3.
anterior worms are summarized in Table 111. Soi~ietinics 1 1 0 shift occurs (cases 6, 7 and 13 in Table) ; i t is probnblv in such c a w s that the original fission level was also the limiting leval.
The wide distribution of tlic fission zone. ixzpoittd in our previous ~ I K Y ( C h and Pai, loc. c i t . ) , may be cxl)lnincd on the basis of t h r s c ol)wrratioiis. The uppermost and lowermost fission Icvc+ t h m obscrvid wcw’ niwely t hc liinit,ing level and the new level rcspectivcly in the anterior wornis of t.he populat~ion.
L
eD
m
TA
BL
E 1I
I.S
hift
ing
of F
issi
on Z
one
in a
nter
ior
wor
ms
of F
amily
I s
how
ing
the
vari
abili
ty o
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e lim
iting
lev
el a
nd t
he
back
war
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ovem
ent
of t
he (
( N
ew F
issi
on Z
one ” af
ter
the
limit
has
been
rea
ched
.
Cas
e.
1 2 3 4 5 6 7 8 9 10
11
12
13
14
Ori
gina
l lev
el o
f fi
ssio
n zo
ne o
f pr
imar
y w
orm
.
18th
tru
nk s
egm
ent
19th
trun
k se
gmen
t
18th
trun
k se
gmen
t
17th
trun
k se
gmen
t
17th
trun
k se
gmen
t
18th
trun
k se
gmen
t
16th
trun
k se
gmen
t
18th
trun
k se
gmen
t
18th
trun
k se
gmen
t
17th
trun
k se
gmen
t
20th
tru
nk s
egm
ent
18th
trun
k se
gmen
t
20th
tru
nk s
egm
ent
16th
trun
k se
gmen
t
Lim
itin
g le
vel o
f fi
ssio
n zo
ne.
18th
tiw
nk s
egm
ent
18th
trun
k se
gmen
t
16th
trun
k se
gmen
t
14th
trun
k se
gmen
t
13th
trun
k se
gmen
t
18th
trun
k se
gmen
t
16th
trun
k se
gmen
t
12th
tnin
k se
gmen
t
17th
trun
k se
gmen
t
15th
trun
k se
gmen
t
17th
tiw
nk s
egm
ent
12th
trun
k se
gmen
t
20th
tru
nk se
gmen
t
13th
trun
k se
gmen
t
Xnm
ber
of s
ucce
ssiv
e ge
nera
tion
s ex
hibi
ting
shi
ft.
I I 2 R 4 0 0 G 1 2 3 G 0 3
__ -_
- - - -
Lev
el o
f ne
w f
issi
on z
one
aft,e
r bac
kwar
d ju
mp.
30th
tru
nk x
egin
cnt
2lst
tiu
nk s
egm
ent
I eth
trun
k se
gmrn
t
18th
trun
k se
gmen
t
18th
tru
nk s
pgm
mt
“2nd
trun
k se
gmen
t
1 itt
i ti
unk
segm
ent
16th
hu
nk
seg
men
t
ihth
tiu
nk s
ep
rnt
18th
ttu
iik
segm
ent
“1st
tru
nk s
egm
ent
17th
trun
k se
gmen
t
d3rd
trun
k se
gmen
t
16th
trun
k se
gmen
t
Ran
ge o
f di
ffer
ence
in
seg
men
t nu
mbe
r he
twee
n tw
o le
vels
.
.- - .
_-
! Fj
I F
amlli
es
IT, I
V, V
, V
1.
, I I
Pri
mar
y w
orm
..
....
....
A
nter
ior
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m
....
....
..
Pos
teri
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orm
...
....
...
'l'u t
al
....
....
....
..
Pri
mar
y w
orm
..
....
....
A
ntor
ior
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m
....
....
..
Pos
tmio
r w
orm
...
....
...
tota
l ..
....
....
....
__ .-
..~ ...
-.
. - .- . -. - - - -
- . - - - - .
,.
Pri
niar
y w
orm
..
....
...
Ant
orio
r w
orm
..
....
....
P
ostp
rior
wor
m .
....
....
'ro
tlll
..
....
....
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Yri
rnar
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orm
..
....
...
Ant
erio
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orm
..
....
..
E'o
ntvr
ior
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m .
....
....
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ii ..
....
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. . - - - . -
__
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-
.. - -
. .- - -. -
-. ..
:.i
200 J. CHU
‘Cable ZV. bummarises observations on the frequency of fission throughout the range of distribution of fission zones in the 664 worme on which a genealogical analysis was made. In anterior worms thc range of distribution of fission zones may extend from the 11th to the 23rd trunk segment. The slight retro- eedence of the uppermost 1evc.l from the 8th to the 11th trunk segment in this as compared with the former report is probably due only to the difference
Figure 4.
An anterior worm from Family I with four fission zones. Fission zones (2) formed LIUC-
crsrively at a, 6, c and d ; the first fission took place a t scgment 18 and gavo rise to an antmior worm (a) and a primary worm (a). The latter divided at segment 18 giving rim to an antorior worm (b) and a posterior worm (6). The anterior worm from the sccond fission divided giving rim to an anterior worm (d ) and a primary worm ( d ) ; while tho posterior worm from the second fission gave rise to anterior worm (c) and posterior worm (c).
in the total number of worms examined. It is clear that the range of fission zones in both primary and post.erior worms is restricted to segments 16 to 22. This limited range, together with the higher frequencies within t,he same range found for anterior worms, presumably determines the maximum range of the higher frequencies of natural fission already recorded in the previous paper. By plotting the data shown in the last section (Section U) of Table IV we
TA
BL
E V.-A
naly
sis
of t
he fission f
requ
ency
dat
a pr
evio
usly
obt
aine
d on
the
bas
is o
f th
e ra
tio o
f th
e th
ree
aorn
i ty
pes
in t
he p
opul
atio
n ex
amin
ed g
enea
logi
cally
.
Visa
ion
frrq
uenr
p.
Oba
enT
rd t
atel
..
. ...
. .. .
. . ..
...
-
Cal
cula
ted
ante
rior
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ms
.. ..
..
Ca1
1~1t
lntt
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prim
ary
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rin
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. . . .
. C
alcu
late
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ster
ior
won
nfi
. . .
. . .
SHpm
cmt N
o. o
f fi
ssio
n zo
ne.
8 9
10
11
12
13
14
I5
I6
17
18
19
20
21
2.'
23
1 4
16
29
62
114
159
215
262
304
337
301
I47
41
9 I
__
__
__
__
__
--_
1
4 16
29
61
11
4 15
9 21
5 20
4 17
2 13
8 97
41
9
51
1
- - - - - - - -
31
65
128
12ti
A9
30
5 -
__
__
_-
--
27
65
71
78
47
I.'
1 -
I I--
202 Figure 5 .
150 I
L E V F L Of F;SSION Z O N F E>:PRESStD AS .4UMOIR OF T R U h K SEGMFNTS A N l i R l O R T O T k ~ ZONf
(iraph showing the frequency of fimion at successive sogmont lovela in the t h r w worm types and in the population as a whole, based on data collected in Table I V .
Figure (i.
.
LEVEL OF FISSION ZONE EXPRESSED AS NUMBER OF TRUNK SEGMENTS ANTERIOR TO T H E Z O N E
(hniparisuii o f the freclutmcy of firrsiuu at m(:('(asslv(. scbgmc,tit l o ~ e l ~ UI thc genc~alugi(:all~ analywd population (0) with the theorotioal IJinoinial curve calculated from t.ht, mame data (continuous curve).
$ISS1ON IN STYLARIA FOSSULARIS 203
Figure 7.
P
LEVEL ar F!sS1014 ZONE EXPRESSED AS NUMBER OF TRUNK SEGMENTS ANTERIOR TO THE ZONE
Comparison of the frequency of fission at successive sogment levels in a wild population (0 )
with the theoretical binomial curve (continuoue curve).
22
ta
14
14
6
2
LEVEL OF FISSION ZONE EXPRESSED AS NUMBER OF TRUNK SEGMENTS ANTERICR TO THE ZONE
Comparison of the data for frequency of fission at aucrosaivr segment levels in a Wild population and in B gemealogivally an~lysrd population (me text). 0. percentage Ircquency in wild populetioi~ ; + , percentage frequency in geaoalogioally analysed population.
204 J . CEKU
obtain tlic curves shown in fig. 5 . 'I'hc. g ( w ~ a 1 tiwid is siniilar amoiig thc: four curves, but only th: curv(*s for antt.rior wornis and for the total show a close icsrniblaiirc: t,o i Iic CUI'VPS in fig. 6 of our fornicr report.
'rhc th tn givcn ill t,he last sc,c.tion of Table IV have beciti subjected to statistic:al malysis. Plot4iiig :L thtwrct,ical hinomial based on the observrti frecp.ricic.s, we find that t.hc obscvwd frequvncy curw agrcm fairly wcll with the thcorctical curvv (iig. 6). In R siniilar way, the nataral freqwncg curve of the worm i)opulation described iii the previoiis paper, in which we did not distinguish the worm types, can a,lso br comptrcd with t.he ideal binomial curve. On the basis of t.heir genealogical ratios it is possiblc t,o cstimat,e the number of each type presc*nt in tho population of 2000 and the respective contrihut.ion of w c h type to the observed fission frquencies, sincc: it is probable that fission t,akes pli tc~ in uat,iire much as i t does under labor&nry condit.ions. The results of this ca~lcnlation are givcw in Table V.
I n contrast to tlic: gcliei1logicall~ iliialysed frequcwcy curve, t,he natural frequcncy cirrvc~ deviates markedly from t,he theorc~tical binomial curve between segment,s 14 and 18 (fig. 7).
To illustrate niorc clcarly the clifferenct. between the curve for thtt natural populat.ioii ; i i d that for the g(~iica1ogically aiialgscd population, the frequency values ca.11 be conwrt iiit,o t,hcir rcspect.ive percrutages. This has been done arid the rcsultiiig data plotted in fig. 8. It is apparcwt that during the ascending phase, tlic fission frcquericies in thc. natitral ~ m ~ d a t i o n exceed those in t.he genealogically nnalyscd polnilat ion. The two meet. at the 16th segment, after whch thc former falls witliin the. seol)c. of t,he lattm. This difierence will be discussed later.
Discussion. From t,he obscv,v;it iolis rcwrdcd it is clear t,liat tlic. position of the fission
zoiit: iii a ppulation of Stylarici fossulnris is very variable.. The range of varia- tion is duc, 1)rimarily t.o t,he cxist(wcr of tlirc.c ivorni typcas. In the case of the priniar>- and posttkrior \voriiis, fissioii onl>. talies place bct,ween the 16th and 22nd truiik hcgmenth. In the CHS(- of antchrior wornis, howcvrr, because of forwald shift and sulJscqutwt backu.ard jump, t.he fission zone extends 0 1 1 both sides of scpicnts l t i t.o 22 , especially iii t.hc forward direction. Beside this intrinsic difliwnce bct w e i i t h e worni tyI)es, external conditions may also play some pa,rt, as shown by Van Cleave (1937), who found that the position of the fission zone can bt. alterc~d by cwvii~orinierital factors. The influtmce of such factors on fission in Styluria fossuluris has not yct been investigated.
A point which reqiiires further discussion is the fission of anterior worms in thr genealogiral series. Xccordiiig to Stephenson (1930) fission in annelids of the StyZaria type is c1i:traaterizc.d by the fact taliat, the ntw fission zone is one segment nearrr t.hc antc.rior rnd than the fist. In the course of the present study, we were able to confirm Stc.phcmson'a stateincarit. and observed, wit.h successive divisions, a shifting forv i ud of t'hc fission zone, segment by sepnicrit. until a limit, wa.s reachcd. After t h a t , the position of t'ho fission zo~ie was found to shift> t);diwartls in a. ni:iiin(~ similar to that dcscribed by Hempelnianri (1923) in Pristinu longisetu. In St?jlaria, however., this limiting level is not, (:onfined t,o a paiticular segrncmt ; it,s 1)osit ion varies in different individuals. It has been shown previously (Chi1 ant1 Rii. loc. c i t . ) , that posterior regeneration is possible if t.ho anterior fragnic.nt possesses a minimum of six trunk segments. In the presc.iit coniinuriicat.ioii it has bc~31i found that, t hr upprrmost liniit of the fission zonc o(:ciirs at, the 8th ti.rink scynicmt. Thc. minimal number of trunk segnittiits which a cla~iiphtt~ antcrior ivorni owrs t ht? niot,her worm is therefore right.. ii valiiv wliic*Ii a~qiroxiniittc~s c*lost*lj- 1 . 0 1 lit. minimal nunibrr of' trunk scgmcmt.s n(x ivy for the rcyrnerat,ion of a \vholc* ~ v o r r i i from an mterior fragmcnt. In othcv words, th* niininial riuni1)ci. of scymc~iits iequircd either for regeneration or for nat.ural fission is ncarly t,he same. But the data given
FISSION IN STYLARIA FOSSULARIS 206
in Tables I11 and I V intiica.te that the limiting level may occur a t any segment hetween the 1 l th ant1 thc ?Ot.Ii, a fact whicli in;i.kw t;liis corrc.sl)onclt.rtc.(. lcss striking. It is clear that thc significii.tt(xL of tlir. liiiiitiiig Irivrl is a ItliLtter for further investig a t ' ion.
The observed tlifferenoc: I)ctwcen t Iiv ciirw*s sltowiitg frcqiwi~cy of fission a t different levels in a natural poj)iiIation a i i d i n t,ho pc.liea,lofiicilll. ;rrialpscd population calls for coinmc.rit. 'L'Iit> first (xirw WLS ohtiLin(x1 hy oonsitlvring only those worms that, had fission %on(! a t t,hc timc of c?saniiiii~tioil ; wornis not showing any sign of fission wcrc neglcctc:d. The genealogical analysis has shown, however, that primary and posterior worms have R lower rate of fission and a greater mortality than anterior wornis. This probably also holds good under natural conditions. If this is so the uncounted worms would belong largely to the primary and the posterior groups, that is to say, most of the individuals among the 2000 worms exarninod were probably of the anterior type.. On tho other hand, in tlic Mi4 geriealogicnlly analysed worms, we are d e a h g with all three types. It is obvious t.li;it., in a natural population, fre- quencies of fission anterior to the 16th trinili wgment can only he attributed to anterior worms ; while posterior to that sqynent all t.hree types are con- tributing to the observed frcqueneic.s. The higher frequericics manifested up to segment 16 in the ascending limb of tho natural frequency curve are due to the fact tha t . more anterior worms were esamined in this case t,han in the genealogically analysed population. t'ostwior to segment 16 the frequencies in the natural population arc corisistc~iitly I)c~low those for the gcncalogically analysed group. This is prcwm;tI)ly clrw to the fact that a proportion of primary and posterior worms will It:tw~ Iwcii neglected l)c~:ausc of their lower fission rate and higher mortillity.
From the above analysis, it is h t r that t lie variation in fission frequencies a t different body IeveLs in Stylariu fo.ssulnris is an instance of coni1)lex variation determined by the combined action of three genealogical worm t y p . The factors responsible for this variation and for the differentiation of the tlirce worm types are unknown.
In Stylaria fossularis the number of segments varies from individual to individual in both asexual and sexual phases. In the asexual stage, t,hc total number of segments in a given worm is variable owing to intirfinit.e posterior growth. This inconstancy not only implies differences between individuals, but also variability in the nuinbw of segments in a single worm at ciiferent growth stages. As a worm reaches t,he sexual st,age, posterior growth ceases and the number of Jegments bcconios fixed a t a definite number, varying among different sexual individuals from 34 to 55. KO c1efinit.e relation between the level of fission in the last asexual stage and the number of segnients of the following sexual genera.tion was observed. Because of this variation in the total number of segments, the fission levels in different worms are not necessarily eompara.ble, even though the serial numbers of the segments a t which fission occurs are the same. I t is probable that t.he observed morphological variation in the position of the fission zone has its physiological background. yurther investigations will deal with this question.
Summary . 1. Worms produced by fission in Stylaria fossuhris may be classiticd into
three types difiering from each other in cwtain ternl jorary niorpliological features.
2. The wide raiige of distribution of the fission zone (from segmcnts 8 to 23) is mainly due to the peculiar behaviour of the anterior worn1 type.
3. The comparatively higher frequcvq of fishion between triiiik ~cymcnts 16 to 22 in a wild population m;iy I ) ( > attt il)iitcd to I lie combiiird eflcct o f iinterior, primary iLnd posttcv%)r M O I ' I ~ I tyl)es.
20G FISSION IN STYLARIA FOSSULARIS
4. The difference between the curves for fission frequency in relation to segment number in a wild population and in a genealogically analysed group is partly due to the uncounted primary and posterior worm types and partly to the higher mortality of the latter under natural condit,ions.
5. The fission frequency curve is a complex variation polygon with three genealogical types of worm as variables.
Literature cited. CHU, J . , & Par, S. The rulations between natural fission and regeneration in Stylurin
foambaris. (In press.) HEMPELWN, F. (1923). Kausal-analytische Untersuchungen iiher daa Auftratun ver-
grossortar Borstun ilnd die Lage dcr Trilungszone hei P r i - ~ t i ~ n . Arch. naikr. An&. 98, 379.
STEPHHNSON J. ( 1930). 7'he Oliyochlrrtu. Oxford University Press. VAN CLEAVE, C. L). (1937). A study of the procaa of fission in t,ho naid Prisli?m lon!/i.vefn.
