Symptom Expression of Fusarium Wilt Disease of the Gros MichelBanana in the Presence of Radopholus similis (Cobb, 1893)
Thorne, 1949 and Meloidogyne incognita acrita Chitwood, 1949
CLIVE A. Loos*
The association of nematodes and microorganisms in plant diseases is not anew concept. Sasser et al (1955) demonstrated the relationship of a root-knotnematode, Meloidogyne incognita acrita Chitwood, 1949, to black shank dis-ease of tobacco. Reynolds and Hanson (1957) indicated that the incidenceof postemergence damping-off disease of cotton, caused by the fungus Rhizoc-tonia solani Kuehn, was affected by the same nematode. Holdeman and Gra-ham (1952) showed that the sting nematode, Belonolaimus gracilis Steiner,1949, facilitated the development of Fusarium wil t both in susceptible andresistant cotton. Lucas et al (1955) found that Meloidogyne incognita acritaas well as mechanical wounding increased the symptom expression of Gran-vill e wil t of tobacco. Stewart and Schindler (1956) found that five root-knotspecies or the ectoparasitic nematode, Helicotylenclms nannus Steiner, 1945,increased the rate of carnation wil t in the pi'esence of the bacterium, Pseudo-monas caryophylli Burk. Crosse and Pitcher (1952) inoculated strawberryplants with a combination of one or other of the nematodes Aphelenclioidesritzema-bosi (Schwartz, 1911). Steiner and Buhrer, 1932, or A. fragarieRitz. Bos, 1891) Christie, 1932, with the bacterium Corynebacterium fascians(Tilford) Dowson and caused typical symptoms of cauliflower disease onthe plants.
Fusarium wil t disease, or the more commonly termed 'Panama disease' ofbananas, caused by the fungus Fusarium oxysporum Schlecht. f. cubense(E. F. Sm.) Snyder et Hanson, is responsible for widespread destruction ofGros Michel variety bananas. A nematode survey of disease-ridden areas inthe Bocas District of the Republic of Panama showed that the burrowingnematode, Kaclopholns similis (Cobb, 1893) Thorne, 1949, which occurs inlesions formed in the cortical tissues of the roots and rhizomes, was the mostcommon plant-parasitic form present. Also a common occurrence, but lessfrequently encountered, were the root-knot species, Meloidogyne incognitaacrita and M. arenaria (Neal, 1889) Chitwood, 1949, which usually formsmall galls at the tips of the smaller roots and occasionally large galls, % to% inch in diameter, in the larger roots. Many ectoparasitic nematodes werealso observed in soil and root washings, principal among them being Heli-cotylenchus erythrinae (Zimmerm. 1904) Golden, 1956, //. mnlticinctus (Cobb,1893), Golden, 1936, Hemicycliophora and Paratylenclms spp. Bacteria-consuming forms of the genera Cephalobus, Rkabditis, Panagrolaimus, Acro-beles as well as the predatory Mononchus spp. were commonly encountered.
The two endoparasites, K. similis and M. incognita acrita, were selected forfurther study as possible agents in the spread of Panama disease because oftheir widespread distribution and the frequency with which they are foundin the root system of Fusarium-wilted plants.
Rhizomes from which the Gros Michel banana is propagated may carry7?. similis in reddish-brown to black lesions up to an inch or more deep intothe soft cortical tissues. The roots may carry R. similis, root-knot and ecto-parasitic nematodes.
'-Plant Pathologist and Nematologist, Dept. of Plant Pathology, Changuinola Research Sta-tion, Chiriqui Land Company (a subsidiary of United Fruit Co.) Almirante, Rep. of Panama.
I am indebted to Dr. 1). Mason of the Institute of Statistics of the North Carolina StateCollege for his interpretation of the statistical analyses. To Mrs. C. A. Loos grateful thanks»TC offered ior help and encouragement during the progress of the experiments and in writ ingthe manuscript.
Loos (1957) and Stover (1957) suggested that root injury aided infectionwith F. oxysporum f. c-ubense. Stover demonstrated that injury to the rootsystem by trenching in the presence of diseased material encouraged the earlydevelopment of disease symptoms. Xewhall (1958) endeavoured to prove arelationship between F. oxysporum f. cubense, the burrowing nematode andunidentified root-knot species collected from galls on tomato plants growingin the fields. He used three-foot diameter tanks to which he transferredrooted banana rhizomes and continued to replace casualties, for some con-siderable time, with more rooted plants. Those transfers and replacementsAvould have caused damage to the roots, a. condition which predisposes the rootto infection with the fungus. Practices by which the plant roots could bemechanically injured were, therefore, eliminated in the experiment describedin this paper.
MATERIAL S AND METHODS
One hundred and forty six concrete tanks, 18 inches in diameter and 24inches high, were placed on a cement floor in three rows of 50, 50 and 46tanks respectively. A layer of horticultural asphalt was applied to the insideof each tank and a four-inch layer of washed stone rubble (% to 1% inchesin diameter) was placed in the bottom to facilitate draining. The tanks werefilled with light sandy-loam soil sifted through a one-quarter inch mesh screenand well mixed. The soil pH was 6.2. The soil in the tanks was fumigatedAvit h Dowfume MC2, at the rate of one pound of fumigant to four tanks.The soil Avas found to be fungus-sterile and nematode-free after fumigation.
The experiment Avas set up for the following treatments:Treatment A. R. similis plus F. oaysporitm f. cnbense.Treatment B. E. similis alone.Treatment C. M. incognita acrita plus F. oxi^pontm f. cnhense.Treatment I). M. incognita acrita alone.Treatment E. F. o;>'i/sporum f. cubense alone.Treatment F. Check:: No fungus or nematodes.TAV O Aveeks after .fumigation the tanks Avere planted Avit h nematode-freed
rhizomes, each under one pound in Aveight. This planting enabled a build-upof the desired nematode population in the appropriate tanks and also alloAvedtime for a reversion of the fumigated soil to.a more balanced soil floral status.Radopholus infected tissue Avas obtained from banana rhizomes from areaswhere Panama disease Avas not abundant. T/he rhizomes \vere Avell Avashedand then trimmed of roots to eliminate both root-knot and ectoparasiticnematodes, after Avhich a thin layer of the rhizome Avas cut aAvay to exposethe typical Radopholus lesions. These lesions Avere cut out and chopped intosmall pieces. One-half pound of this inoculum, Avhich contained about 30,000nematodes and an undetermined number of eggs, Avas dug into >the soil aroundthe recently planted rhizomes in the 50 tanks of treatments A and B. M.incognita arn*d-infected banana roots Avere washed, surfade-sterilized for 5minutes in 10 per cent Clorox (containing) :0.525% sodium hypochlorite) andcomminuted in a. Waring Blendor. The'-'comminuted material was dilutedAvit h Avater to facilitate handling and equal portions Avere added to the soilaround the planted rhizomes in tanks of treatments C and D. At the sametime four tomato seedlings Avere planted in those tanks to 'build up a root-knot population. Five Aveeks later the tomato plants Avere cut back to collarlevel, the banana plants in all the tanks Avere. uprooted and the roots Avereincorporated back into the soil. The rhizomes, many of Avhich carried severeinfections of a bacterial rot, Avere discarded. :
Table 1. Number of plants showing severe head rot, germination failure and thef inal total of replicates in each treatment.
Initial Finalno. of Cases of Germination total of
Treatment replicates severe rot failures replicates
ABCDEF . ..
Total
252525252521
140
3
44411
17
10
111.)
8
211920202:-}18
121
"Button seed" plants for the final plantings were grown from large rhi/omesections or "bullheads" (Anon. 1957) off plants that had produced fruit . Thebullheads were washed, trimmed of all roots, and then cut into discs contain-ing about four inches of the rhizome and six inches of the pseudostem. Thesediscs were placed S to 10 inches apart on a cement floor, covered with saw-dust and kept damp. The "buttons" developed into vigorously growing plantsafter about two weeks. When these plants were about 2 feet high and hadformed a rhizome about 1% Ibs. in weight they were ready for use as "seed."The plant was cut away from the disc, the rhizome trimmed of roots and dis-colored tissue and the pseudostem cut back to within 2 to 3 inches of therhizome. The trimming caused a shellac-like exudation which set hard andformed a protective covering around the trimmed tissues. These nematode-freed seed were air-dried for 24 hours before they were planted in the tanks.The tanks were weeded every third day throughout the course of the experi-ment to avoid root disturbance of the growing banana plant.
After seven weeks, a period sufficiently long for a build-up of nematodepopulations and their resultant damage to the roots, the tanks in treatments A,C and E were inoculated with a culture of F. oj't/Hporum f. cube-use. The 8week old culture was grown in sawdust-cornmeal (5%). One hundred andeighty-five grams of inoculum, each gram containing about ten million spores,was added to the soil surface and the tank was watered heavily to encourageseeping in of the spores. When the excess water drained, one-half inch ofsterilized soil was added to the top of the tank. A second inoculation Avasmade four weeks later using one pound of a 4 week old sand-cornmeal (5%)culture. In this case, too, the inoculum was spread on the soil surface andthe tank watered heavily. This second inoculation brought the total sporeapplication to about 14 million per square inch of soil.
RESULTSTanks in which rhizomes failed to sprout or where rhizome rot was so severe
as to cause undesirable complications in the expression of results were notincluded in the experiment. However, 19 cases of very light rhizome rot Avereincluded. The number of replicates in each treatment at the commencementof the experiment, cases of seA'ere rhizome rot, and the final total of replicatesare summarized in Table 1.
Twenty-three plants were Avashed from the tanks when they were about 14Aveeks old (7 weeks after inoculation with the F-usarimn culture). Al l of fiveplants examined in the A treatment (.K. similis plus Fusarhim) Avere severelyAvil t diseased; IAV O were diseased and tAvo healthy in the C treatment (Meloi-dogyne plus Fusarium} ; four were diseased and tAvo healthy in the E treat-ment (Fusariiim alone).
TABL E '2. Results of treatments. Plants about 19 weeks old and 12 weeks af terfirst, and 8 weeks after the second, inoculation with Fusariuin wil t fungus.
Treatments
A. Radopliolus similis plus fungusB. Rado'pholitti alone0. Meloidogyne incognita acri-ta
plus fungus -D. Mt'loi-doqyne aloneK. Fungus aloneF. Check. No fungus or nematodes
No. ofreplications
1(515
1619191 H
No. ofFitsarium-
wilted plants
160
160
180
No. of plantsfree of
Fusari-um wil t
015
0161
13
Table 2 shows the results of the f inal examination when the plants wereabout 19 weeks old. Treatment A, C and E had received two inoculations withthe Fusarium fungus. At the high level of inoculum of 14 million spores persquare inch of soil all plants except one in the series to which fungus inocu-lum was added, were wil t diseased. None of the plants in the series B, D andF which did not receive the Fusariiim inoculum became diseased.
TREATMENT A., 7i'. Kim His plus F. o.ri/.^pontin f. riihensc-: A marked featurewas the rapidity of infection and the severity of disease expression. Four ofthe 16 plants became visually diseased only six weeks after inoculation withthe fungus. Two weeks later all the plants were severely diseased. Thepseudostem split longitudinally from just above the collar, the split extend-ing 6 to 14 inches up the plant (Fig. 1-B). The lower leaves yellowed pre-maturely and the typical Panama disease fiber discoloration, seen in crosssection of the leaf sheath, was a marked feature. The characteristic brown-ish-red discolorations of the vascular tissues of the rhizome were very promi-nent (Fig. 1-D).
DFig. 1. A. Gros Michel banana plant, 14 weeks old and 7 weeks after inoculation
with Fittiaritnn o.vi/fi))orio)i f. cube-nxc- culture. Although the rhizome was diseasedthere were no wil t symptoms on the aerial portion of the plant. B. Same age asA. Inoculated with Radopliolitx til ni Hi ft in addition to F. oxijtipontm f. cultcnsc.Note the severe splitting of the pseudo-stem which was a characteristic earlysymptom of wil t disease in the presence of that nematode. C. Rhizome of a checkplant, 14 weeks old, split to show healthy tissue. Note profuse root formation.D. Split section of rhizome of plant shown in Fig. B. Note the wil t disease symp-tom shown as severe discoloration of the stelar region.
TREATMENT C., M. incognita acrita, plus Fiisariirm : The severe diseasesymptoms of Fiixuriirni wilt , expressed as premature yellowing of the leavesand splitting of the pseudostem appeared on only a few plants. The timeinterval between inoculation and symptom expression was longer than inTreatment A. Five of the 16 plants developed severe visual symptoms 10weeks after inoculation. Finally, 8 of the 16 plants became visually diseased,though at the end of the experiment all 16 carried disease symptoms in therhizome.
TREATMENT E., F. o.t'i/nponint /. cubenxe (done: The tanks in this seriescarried healthy looking plants, except for three plants which developed thecharacteristic splitting of the pseudostem. Disease expression developed 10weeks after inoculation. Many of the rhizomes showed only a trace ofFnxarium infection; in many of the plants it was not possible to diagnosethe wil t disease from visual appearance of the aerial growth (Fig. 1-A).Diagnosis was dependent on examination of split rhizomes. Several isola-tions were made from diseased material and the isolates identified as F.o.vjixporum f. eubenxe.
TREATMENTS B, D, AND CHECK F: Xot one of the 47 plants not inoculatedwith Fusurium developed wilt-disease symptoms. The roots of the checkplants were free of nematode infections (Fig. 1-C). There was no doubt,from the results of these examinations, that fumigation of the soil and trim-ming of the rhizome effectively removed fungus and nematode contaminations.
DAMAG E ATTRIBUTED TO XEMATODESTable 3 shows the growth of plants 13 weeks old, measured from collar
level to the fork at the two upper-most opened leaves. The addition of theFiiHdriiiiit to plants already infected with a specific nematode did not affectgrowth, nor was there a difference between check plants and those inoculatedwith the fungus alone.
Table 3. Growth of Gros Michel banana plants over a 13 week period as affectedby inoculations of: Radopliolim similitt plus Ftixariitm o.vi/sponrin f. cubcnuc fungus( A ) ; R. fii.miliH alone (B) ; Meloidogi/nc incognita acrita plus F. oxysponim f.cubcnse (C) ; M. incognita acrita- alone (D) ; F. oxi/tipornni f. citbcnse alone (E) ;
and Cheeks (F).Treatments .No. of replicatesMean height (inches)SxO ~\ ( ®/n ^
A2118.00.360 9
111919.10.430 10
C1915.70.580 16
I )211-1.90.630 19
E2320.80.540.13
F1819.30.680 15
There are, however, measurable differences between the M. incognita acrita(C, D) and the nematode-free series (E, F). There is no doubt that thegrowth of these nematode-free plants was superior to that of the Me.lwla-gyne infected plants (Fig. 2-A) and that such difference is real (P<.()1).Growth differences of considerably smaller magnitude but still statisticallysignificant were observed between Meloidogi/ne (C, D) and lladoplwliis (A,B) treatments and also in the comparison of the Radopholtis (A, B) with thenematode-free (E, F) treatments. At the final examination of the survivingplants, after 19 weeks growth, the severely wilt-diseased plants, in treat-ments which had Fuxari'inn inoculations, had ceased to grow. However, thesignificant difference between growth of Meloiflogi/nc infected (treatment D)and the check plants was maintained, though visually it was not so apparent
as at the observation at the 13th week. On the other hand, the Radopholusinfected plants (treatment B) were measurably smaller and showed a signifi-cant fal l ing off of growth in comparison with the checks. At the end of theexperiment (19 weeks) the nematode infected plants were obviously smallerthan the checks.
R. similix infection caused severe root destruction and muti lat ion (Fig.2-B). The roots still attached to the rhizomes carried numerous lesions, manyof which were up to 2 inches long and had completely girdled the root.
TABL E 4. Weights of roots and of total plants in treatment A (Radopholus tiimilifiplus Futiariiim oxyxponnii f. cnbcn.nc) and E (F. o.ri/xponim f . citbenxc alone) at
terminat ion of experiment. Plants 19 weeks old.Eoot Weigh IN To/al Plan t Weigh IK
Treatments ..Xo. of replicatesRange in weights (ozs.)Mean weight (ozs.)SxC. V. (%)
A158-29
22.71.360.23
E1519-5(536.5
2.820.30
A1596-132
118.93.060.10
E15
104-2161544
8.560.21
The lesions contained large numbers of Radopholus adults, la rvae and eggs.The type of damage was typical of that described for field conditions (1957).The small roots, so abundant in the series of plants which were nematode-free, were almost completely absent or dead in the Radopholus infectedplants. Table 4 shows the comparative root and total plant weights of treat-ment A (A'. KIM ills plus F. oxpspontm) and treatment E (F. oxyspornmalone). The mean weight of the nematode-free plant roots is considerablygreater than that of the Hadopholus infected plants.
Table o shows the number of living roots over 3 inches long, on plants oftreatment A (Radopholus plus Fusarium); B (Meloidogyne plus Fusarium)and E (Fitxariinii alone), counted when plants were 19 weeks old. The meannumber of roots on plants in the Radopholus series is about one-third of thatin the Meloidogyne infected plants and about one-fifth that of the nematode-free series.
Table 5. Number of living roots present on plants of treatments A (Radopholussimilifi plus F-imarium o.vi/xporum f, cubcnsc), C (Meloidoffyne incognita acritaplus F. oxysporum f. cubc-ntse) and E (F. oxyxporinm f. cube-nxc alone). Plants
examined when 19 weeks old.Treatment A C EXo. of replicates 12 13 13Range in number of roots 12-59 59-85 87-200Mean of root numbers 28.8 70.9 139.5Rx 4.35 2.5 9.6
The number of roots present in the Radopholus infected plants is significantlysmaller than that of the Meloidogyne infected plants; in turn the root systemof the Meloidogyne infected plants is smaller than that of the nematode-freeplants. The differences are highly significant (P<.01).
DISCUSSION
Infection of Gros Michel plant roots with either R. similis or M. incognitaacrita was not a pre-requisite to wil t disease infection when the F. oxusporumf . cubense fungus inoculum was at a high level. When the plant roots were
Fig. 2. A. Comparative growth of plants 11 weeks old, infected with Meloidogyneincognita, acrita and nematode-free. Plants in the row on the right of the picturewere lleloidogyne infected. B. Eoot systems of Radopliolus slmi.lix infected(right) and Nematode-free check (left) plants. Plants 19 weeks old. 0. "P>uttonSeed" rhizome pared of outer tissue to expose P. HI mills lesions in the rhizomecortical tissues.
heavily infected with R. similis, the disease expression was aggravated andthe period between inoculation and the appearance of marked disease symp-toms was considerably shortened, though the final result of disease incidencewas not affected. Unfortunately littl e is known of the lowest spore popula-tion in the soil capable of producing disease symptoms in banana plants.Stover (1957) considered that populations below 500 spores per gram of soilcannot be accurately determined by soil dilution plates and that the low con-centrations of spores in soil makes detection on plates remote. Fourteen mil-lion spores to a square inch of soil in fairly even distribution over a confinedsurface is extremely heavy inoculum. Even under those conditions the dif -ference in rapidity of onset of disease symptom expression in the presenceof R. si-milis was most marked.
In these experiments, the destructive nature of R. si-milis on banana rootswas manifested. The depleted root system of heavily infected plants must re-strict the assimilation of plant nutrients as well as weaken the plant's anchor-age. The result of this inadequate anchorage becomes abundantly evident oninfested fields as plant 'uproots' or 'tip-overs.' The pull exerted by theweight of a developing fruit or the light push of a gust of wind is oftensufficient to uproot the plant. On the other hand, the profuse unleskmedroot system of nematode-free plants must contribute effectively to the settingof large fruit and a stand of plants which would not uproot except underextremely heavy gusts of wind.
SUMMARY
Infection of Gros Michel banana roots with either the burrowing nematode,RaflopiioliiH ni-m ills or the root-knot nematode Meloidogyne incognita acrituwas not a prerequisite to wil t disease infection when spores of Fnsariumo.i'ijxpornm f. ciibetixe, from culture, were added at the inoculation level of 14millio n spores per square inch of soil and drenched in with water. When theplant roots were heavily infected with 7?. si-mil is, the period between inocula-tion and appearance of disease symptoms was considerably shortened, thoughfinal result of disease incidence was not affected at this high inoculum level.
7?. xw///.y caused severe lesioning and destruction of banana roots. Theroot system was severely depleted and plant growth affected. The nematodecaused large lesions which often girdled the root and extended through thecortical tissues to the stele.
LITERATURE CITED
ANON. 1957. Plant parasitic iiematodes and their association with bananas.Uni ted Fruit Co. Res. Dept. Ext, News Letter 4: 8-1 (i
ANOX . 1957. Banana " Button Seed." Ibid. 4 (3 ): 1-8.CROSSE, .T. E. and PITCHER, R. S. 1952. Studies in the relationship of eelworms
and bacteria- to certain plant diseases. 1. The etiology of strawberry caul i-flower disease. Ann. Appl. Biol. 39: 475-48.
HOLDE.MAX , Q. L. and GRAHAM, T. W. 1952. The association of the sting nema-tode w i t l i sonic persistent cot ton wi l t spots iu northeastern South Carolina.P l i y t o p a t l i: 42: 28:5.
Loos, C. A. 1957. Annual Reports. Clianguinola Research Station. United FruitCompany L ibrary. Unpublished.
LUCAS, G. B., SASSER, J. N. and KELMAN , A. 1955. The relationship of root-knoti iematodes to Granvi l le wil t resistance in tobacco. Pl iytopat l i: 45: 537-540.
N E \ V " - I . L . A . (i . 1958. T:!-.> incidence of Panama disease of bananas in the pres-ence of the root-knot and the burrowing iiematodes (Mel&idogyne and Jia-cloplioliia'). Plant Disease Reptr. 42: 853-850.
REYNOLDS, II . W. and HANSON, R. C. 1957. Khizoctonia disease of cotton in])roscnce or absence of the cotton root-knot nemntode in Arizona. Phytopath.47:256-261.
SASSEB, J. N., LTCAS, G. R. and POWERS, H. R., JR. 1955. The relationship ofroot-knot neniatodes to black-shank resistance in tobacco. Phytopath 45: 459-461.
STEWART, E. N. and SOHINDLER, A. F. 1956. The effect of some ectoparasitic andendoparasitie neniatodes on the expression of bacterial wil t in carnations.Phytopath. 46: 219-222.
STOVFR, R. 11. 1956 . Studies on fusarium wil t of bananas. 1. The behaviour of/*'. o.i'i/spornm f. cubensc in different soils. Canadian J. Bot. 34: 927-942.
STOVER, E. H. 1957. Progress report of studies and observations on the behaviourof FitNariitm oxyxpornm f. cnbcnaa in bananas. United Fruit Co. Library.Unpublished.
The Histology of the Prostate Mass in the Genus Acanthatrium(Treiiiatotla: Lecithodendriidae) .*
THOMAS C. CHENG**
The members of the trematode genus Acantliatrium Faust, 1919, can berecognized by the presence of a genital atrium lined and/or bordered withspines and numerous prostate cells surrounding the atrium. The other genericcharacteristics were given by Cheng (1957) in the revised form. In the samepaper Cheng pointed out that the main criterion for distinguishing this genusfrom the related genera, i.e. Pj/ciioporni* Looss, 1899; Prosthodendrium Doll-fus, 1931; Lecitlioflaidriiim Looss, 1899; Paralecithodendrium Dollfus. 1931;Phaneropsolm Looss, 1899; and Parabaxcn* Looss, 1907, all of which are in-testinal parasites of mammals, is the presence of the spinous atr ium. Thepresence of th prostate mass also occurs in the first four related genera. Thispeculiar g landular mass, which is observed to li e dorsad to the genital atrium,undoubtedly serves the same function as the prostate glands which are usuallyfound enclosed within the cirrus pouch in trematodes which possess this latterorgan. The histological make-up of the prostate mass has never'been reportedand because of its peculiarity, Avarrants investigation.
Several specimens of AccmtJiatrium pipixlrclli Mai-y, 1940 and A. nligacan-tlnnii Cheng, 1957, recovered from the ir bat hosts in Albemarle County, A7ir-ginia, were fixed in Carnoy's (6:1:1), sectioned at 10 microns in thickness,and stained with Mallory's Triple.
HISTOLOGICAL OBSERVATIONS
Observations on sections cut through the level of the genital atr ium andprostate mass in both species revealed a similar arrangement of the internalstructures but with some variations. The interior of the atria 1 wall is linedwith spines. In A. pipiKtrelli this wall, measuring 0.008-0.Oil mm. in thick-ness, is of a homogeneous, non-cellular nature and stains the same color asthe atrial spines. In A. oligacanthum the wall is thinner and less obvious.
'•'Contribution from Mountain Lake Biological Station, University of Virgini a and thedepartment of Histoloffy and Kmbryology, Dental School, University of Mary land. The re-search was made possible (in part) through the National Science Foundat ion Award (Grant-2 0 H 6 ), Summer, 1958.
**Present address Department of Histology & Embryology, Dental School, Univers i ty ofMaryland, Baltimore, Mary land.
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