and P series to lag behind the other cells. The SI cell 

 group again divides, A and b on the left side, a and b on 

 the right side, each giving rise through an oblique divi- 

 sion to two cells, making four on each side of the body. 

 At about the same time P2 undergoes an unequal division, 

 the larger daughter cell (the third somatic stem cell, S3) 

 being dorsal, and the small germ cell (P3) being ventral; 

 the ventral cells MSt and E both divide, MSt longitudin- 

 ally giving rise to MST (left) and mst (right), and E 

 dividing transversely gives rise to E I (anterior) and 

 E II (posterior). The posterior dorsal cell, S3 like SI 

 is an ectodermal stem cell. It divides longitudinally 

 forming C (left) and c (right) ; these cells produce the 

 hypodermis of the posterior part of the embryo. The 

 germ cell P3 again undergoes unequal transverse divi- 

 sion, forming a larger dorsal cell, Si, and a smaller ventral 

 cell, P4- 



These cleavages, four in number, bring the embryo 

 to the 16 cell stage at which time all of the somatic 

 stem cells have be^n formed (Fig. 152 F). The embryo 

 is an elongate blastula with an inconspicuous blastocoele. 

 Cells destined to form the ectoderm cover the anterior 

 and dorsal surfaces of the embryo, cells destined to form 

 the mesoderm and entoderm cover the posterior ventral 

 surfaces. In later cleavages it is somewhat easier to 

 follow the fate of the cells of each stem line separately. 



At the 24 cell stage SI consists of 16 cells (Fig. 152 

 G-H) : six on the left side of the embryo, four in the 

 center, and six on the right side. The origin of the 

 mediodorsal row of four cells has not been determined. 

 Between this stage and the 141 cell stage (Fig. 152 I-J) 

 gastrulation is completed; descendants of SI come to 

 cover the anterior two-thirds of the embryo, the cells 

 small and numbering- 115. The remainder of the embryo 

 is covered by descendants of S3. In further development 



51 comes to make up four-fifths of the hypodermis, gives 

 rise to the nervous system and in postembryonic develop- 

 ment to the vulva (vagina). 



Returning to the S2 line of somatic cells, the four 

 cells present in the 16 cell stage, MST, mst, E I and E II, 

 proceed at unequal rates. At the 26 cell stage there are 

 four cells derived from MSt, namely, ST, st, M, and to, 

 formed by transverse division of the bilaterally sym- 

 metrical cells mst and mst. By the 82 cell stage the six 



52 cells form a total of ten, in paired bilateral rows of 5 

 cells as follows: ST, st, M I, to 1, M //, to //, E I, e I, E //, 

 e II. At this time the entire S2 cell group is somewhat 

 sunken inward, making a ventral groove which is com- 

 pletely covered by ectoderm at the 141 cell stage, and 

 gastrulation is completed. By the 171 cell stage there 

 are four cells derived from ST and st, eight formed from 

 M, and seven from E. Shortly thereafter there are eight 

 entodermal cells. The M line (mesoderm) lies in the 

 body cavity. By the time the embryo takes a definite 

 vermiform shape (Fig. 152 O) there are 12 entodermal 

 cells (£■). 



At the termination of gastrulation or the 141 cell 

 stage, the third somatic stem line, S3, consists of 11 cells. 

 The first stage larva, at hatching, contains 15 cells of 



53 origin which form approximately one-fifth of the 

 hypodermis, for they cover dorsal-posterior, and postanal 

 parts of the body. 



The fourth line of somatic stem cells, Si, which origi- 

 nated at the fourth cleavage or 16 cell stage consists of 

 four cells at the termination of gastrulation. It forms 

 the tertiary ectoderm (Ec III) which gives rise to the 

 proctodeum or rectum. 



The germ cell line represented by Pi passes a quiescent 

 stage during gastrulation. A cleavage takes place shortly 

 before the 171 cell stage forming two cells, one of which 

 (P5) produces the reproductive cells, while the other (S5) 

 produces the somatic part of the reproductive system. 

 Shortly before hatching both P5 and S5 divide, forming 

 S5 I anterior, S5 II posterior, G I and G II. The 

 S5 cells surround the P5 cells and are generally termed 

 "terminal' or "cap" cells. In the male the entire vas 

 deferens and seminal vesicle are formed later on by 

 S5 I, while S5 II forms the epithelium of the testis. 

 In the female S5 I forms the somatic part of the ovary 

 while S5 II forms the oviduct, uterus, and seminal re- 

 ceptacle. 



The entire development from fertilization to formation 

 of the larva within the egg requires two days. The larva 

 is "born" three days later. At this time the larva 



possesses the same number of cells as the adult in all 

 systems except the hypodermis and reproductive system. 

 The female is mature on the 6th to 7th, the male on 

 the 9th day after birth; specimens of both sexes may 

 live 49 and 48 days respectively. 



The gastrulation being somewhat atypical, there is 

 difference of opinion as to the names of germ layers to 

 be applied to the various somatic stem cells. Pai re- 

 gards the St cell group which later forms the esophagus 

 as secondary entoderm but since it is comparable to the 

 M cell group it is better termed mesoderm. The S5 

 group, forming the somatic part of the reproductive 

 system, he also terms entoderm, though mesoderm would 

 appear preferable. Study beyond the vermiform or "tad- 

 pole" stage (Fig. 152 0) is difficult and thus far has 

 been carried out only by means of totomount preparations 

 which lends uncertainty as to the results. The following 

 is a catalogue of the cells of the adult. 



Table 8. Derivation of cells in Turbatrix aceti. 



Number of cells 



Stem cell Structure 



51 4/5 of ectoderm — 



Nervous system. 



Dorsal anterior to nerve ring 23 



" posterior to nerve ring 9 



above bulb 5 



" subdorsal cephalic ganglia 2 @ 6 12 



Lateral ganglia 2 @ 28 56 



Ventral subventral ganglia posterior to nerve 



ring __ 38 



retrovesicular ganglion 17 



anterior to nerve ring 13 



" subventral cephalic ganglia 2 @ 7 14 



" nerve 64 



Excretory cell _ 1 



52 (EMSt) 

 Esophagus 



Corpus 35 



Isthmus 



Bulb -- 24 



Esophago-intestinal valve — 5 



Intestine 18 



Musculature 64 



Connective tissue _ 16 



53 (Secondary ectoderm) 



Hypodermis of dorsal and postanal regions 



about 5 



54 (Tertiary ectoderm) 



Rectum _ 20 



55 (Only partially determined) 



Seminal vesicle - 24 



Ejaculatory duct — - 8 



Other structures ? 



Rhabdias bufonis. The embryology of this species was 

 described by Metschnikoff (1865), Goette (1882), Neu- 

 haus (1903), Ziegler (1895), and Martini (1907). Of these 

 studies those of Metschnikoff were rather casual. Goette 

 committed an unfortunate error in incorrectly orienting 

 the early stages of the embryo, the anterior end being 

 considered the posterior and vioe-versa. 



The first cleavage of this species differs from that of 

 Turbatrix because it is more nearly transverse, due to 

 the difference in shape of the egg. Ziegler traced the 

 embryology partially through the eighth cleavage. In 

 most respects his results correspond to those obtained 

 by Pai. However, there are some differences. Martini 

 has given more exact data regarding the late embryology 

 than are known in the case of Turbatrix aceti. 



During the early stages, the embryology of Rhabdias 

 is nearly identical with that of Turbatrix. The following 

 differences have been noted: P2 divides (third cleavage) 

 before the fourth cleavage begins in the SI group (Fig. 

 151); S3 divides longitudinally instead of transversely 

 at the fourth cleavage forming C I and C II. At the 

 fifth cleavage the embryo consists of a 30 cell blastula 

 (16 SI cells, 2 St cells, 2 M cells, 4 E cells, 4 S3, Si 

 and Pi cells. The fourth somatic stem cell does not 

 divide until after the sixth cleavage has taken place in 

 the SI group forming 32 primary ectodermal cells. At 

 this time S4 divides longitudinally forming D and d. At 



220 



