final body surface (Fig. 154 JJ-KK). By the end of 

 the seventh cleavage the eight large E cells nearly com- 

 pletely fill the blastocoele. 



At the eighth cleavage all cell groups with the ex- 

 ception of P4 divide so that a 202 cell embryo is formed. 

 its anterior surface is covered by small cells of the SI 

 group while much of the posterior part of the embryo 

 is covered by the larger fc, oy, uy, g and x cell groups 

 (Fig. 154 AA & EE). Cells of the C line (c //' C //' etc.) 

 form paired posterior subdorsal rows of cells while those 

 of the D line (d 11' D //') enter the ventral groove. The 

 St cell group now consists of 16 cells, some of which 

 extend as far posterior as the genital primordium (Fig. 

 154 BB) while the remainder form an anterior groove 

 in continuation with the primary ectoderm (Si). The 

 M cell group consists of two irregular lateral groups of 

 eight ceils each, entirely enclosed as is the 16 cell E 

 group by SI and S3 cell groups. The gastrular cavity 

 is sharply V-shaped anteriad, lined by cells of the 

 St group while it is U-shaped posteriad, the large genital 

 primordium cells (Pl, 1 and /// forming the ventral sur- 

 face (Fig. 154 CC). Parts of the S3 cell group (c // 

 and C II) are definitely mesodermal. 



At the ninth cleavage the embryo is 402-celled, heing 

 composed as follows: SI cell group 256, St group 32, M 

 group 32, E group 32, c / and C / (Secondary ectoderm) 

 group together 16, c // and C // (Tertiary mesoderm) 

 group together 16, Si (D) group 16, and P4 (G) group 2. 

 The two subdorsal surface cell rows (Fig. 154 FF-GG) 

 are formed from c / and C /; two large lateral mesodermal 

 bands are formed from M, c II and c 11 and part of St. 



The anterior part of the ventral groove has completely 

 closed, the esophageal primordium forming a solid cell 

 mass in contact with the ventral and anterior small cells 

 of the primary ectoderm. A terminal cavity, the stomo- 

 deum is then formed in the esophageal primordium. The 

 most posterior St cells (Fig. 154 BB) do not become a 

 part of the esophageal primordium; though at this stage 

 the maximum number of St cells should be 32 and though 

 not all of them enter into the formation of the esophagus, 

 there are about twice that number in the primordium. 

 The cells not accounted for were probably small SI cells 

 which entered the primordium during formation of the 

 stomodeum. 



During the latter part of the ninth cleavage the dorsal 

 cells of the C group come to form a single row of 10 very 

 large cells covering the dorsal and posterior surfaces of 

 the embryo, this being accomplished through median 

 movement of alternate cells (Fig. 154 JJ-KK). Anteriorly 

 this row is continued by the cells designated kar 11 B, 

 kal II A, their sister cells being lateral to them. At the 

 sides of this dorsal cell row there are 2 large subdorsal 

 cell rows formed from the gar, gal, kal, kar, cell groups, 

 and at the sides of these, 2 lateral large cell rows formed 

 from the oy and uy cell groups. These large cells are of 

 particular significance for they swell in size and then 

 cover most of the posterior and ventral cells of the Si 

 group, thus forming the epithelium of much of the body. 



At this time the embryo begins to elongate definitely, 

 and becomes ventrally curved, this probably being due 

 to swelling of the 5 large cell rows. The lateral cell rows 

 nearly tome together, ventrally forcing some of the small 

 superficial cells anteriad. This is considered the com- 

 pletion of gastrulation. The anterior end of the embryo 

 and the ventral surface are covered by small cells of 

 the SI group. We now find the mediodorsal and posterior 

 medioventral parts of the embryo covered by cells derived 

 from S3 ( c I and C I) ; the sides by cells of the Si cell 

 group (oy, uy, kar, kal, gar, gal, uy, and oy) ; and the 



Fig. 154. 



Purascoris cqiioruvi. A — 2 cell stage ; B — 4 cell ; C — 6 cell ; D — 

 8 cell : E — 8 cell ; F — 10 cell ; G — 12 cell, lateral view ; H — 12 

 cell, ventral view ; I — 12 cell, sagittal section ; J — 16 cell ; K — 



16 cell ; L 22 cell ; M — 24 cell ; ventral view ; N — 24 cell, 



lateral view ; O — 26 cell, dorsal view ; P — 28 cell ; Q — 41 cell, 

 dorsal view ; R — 41 cell, lateral view ; S — 44 cell dorsal view ; 

 T — 48 cell, dorsal view ; V — 48 cell lateral view ; W — 54 cell, 

 ventral view ; X — 56 cell, ventral view ; Y — 92 

 cell ventral view ; Z — 92 cell, cross section ; AA — 

 202 cell, lateral view (Sth cleavage) ; BB 102-202 cell, 

 horizontal section (in 8th cleavage) ; CC-EE — 102-202 cell, ventral, 

 ventral, and dorsal views FF-GG — 202-402 cell, cross sections (in 

 9th cleavage); HH — 402-802 cell, ventral view; II — Esophageal 

 region of late embryo ; JJ-KK — Ventral and lateral views of same ; 

 LL — Prelarval stage, surface view. 



A-W, After zur Strassen, 1896, Arch. Entwickelungsmechanik., 

 v. 3 (1-2) ; X-Z, After Boveri, 1892, Sitz. Gesellsch. Morph. & 

 Physiol., v. 8; AA-KK. after H. Mueller. 1903, Zoologica (41). 



anterior and ventral part of the body by Si. A further 

 division at least of the large surface cells takes place 

 after elongation of the embryo into definite vermiform 

 shape. 



Other species. — The embryology of Rhabditis terricola, 

 Diplogaster longicauda, and Nematoxys ornatus is, so far 

 as known, similar to that of Rhabdias bufonis. In Pseu- 

 dalius minor no blastocoele is developed, the embryology 

 being very similar to that of Camallanus lacustris. In 

 the case of Syphacia obvelata (Oxyuris obvelata) early 

 cleavage is somewhat modified through the elongate 

 "banana" form of the ovum. The first cleavage is ex- 

 tremely unequal, SI being nearly twice as long as PI. 

 This type of ovum is very common in oxyurids and 

 thelastomatids. The first cleavage of Metastrongylus 

 elongatus appears equal but must be unequal since PI 

 contains a large amount of yolk material while SI does 

 not. As in Camallanus, no blastocoele develops. 



Abnormal development. Development is strongly deter- 

 minate as would be indicated from the previous discus- 

 sion. Sometimes variations occur in the early cleavages, 

 particularly in Parascaris. Normal formation of rhom- 

 boid embryos in the four-cell stage is assured in most 

 nematodes but in this form, due to the planes of the 

 second cleavage, arrangement of the cells is observed 

 which becomes rhomboid by passing through an 1 -shaped 

 stage. Sometimes however by passing through an ["- 

 shaped stage the positions of the blastomeres are re- 

 versed, B being anterior to A. In such cases the entire 

 development of the embryo is reversed; both A and B 

 develop normally like B and A; the third somatic stem 

 cell is formed at the opposite end of P; S2 divides nor- 

 mally and development proceeds to the blastula stage; 

 development of Mst is probably influenced since gastrula- 

 tion does not occur- Injury of P2 in the 4-cell stage does 

 not stop further development of the SI and S2 cells up 

 to the blastula stage, which is abnormal; injury through 

 loss of cytoplasm in the SI cell at the two-cell stage does 

 not stop further development of the PI cell in a normal 

 manner. The position of the spindle of the first cleavage 

 may be changed through centrifuging or by multispermy; 

 in either case the first cleavage may give rise to equipo- 

 tential blastomeres which result in the formation of PI 

 and SI cells, showing that the potentialities are dependent 

 upon cytoplasmic material and that probably the oc- 

 currence of chromatin diminution in a blastomere is also 

 dependent upon the cytoplasm. Separation of PI and SI 

 in Turbatrix aceti results in the degeneration of SI while 

 PI continues development to the 4-cell, to 16-cell or gas- 

 trular stage. 



These observations appear to indicate that nemic em- 

 bryos are essentially of mosaic structure, and that the 

 unequal potentialities of the blastomeres are due to some 

 differences in the cytoplasm but probably also to other 

 factors such as influence from surrounding cells and 

 differences in chromatin. 



Bibliography 



Auerbach, L. 1S74. — Organologische Studien. Zur Char- 

 akterisiik und Lebensgeschichte der Zellkerne. 262 

 pp., pis. 1-4. Breslau. 



Bonfig, R. 1925. — Die Determination der Hauptrichtun- 

 gen des Embryos von Ascaris megalocephala. Ztschr. 

 Wiss. Zool., v. 124 (3-4) : 407-456, figs. 1^25. 



Boveri, T. 1893. — Ueber die Entstehung des Gegensatzes 

 zwischen den Geschlechtszellen und den somatischen 

 Zellen bei Ascaris megalocephala. Sitz. Gesellsch. 

 Morph. & Physiol., v. 8 (2-3) : 114-125, figs. 1-5. 



1899. — Die Entwickelung von Ascaris megaloce- 

 phala mit besonderer Riicksicht auf die Kernverhal- 

 tnisse. Festchr. Kupffer, Jena: 383-430, figs. 1-6, 

 pis. 40-45, figs. 1-45. 



1909. — Die Blastomerenkerne von Ascaris mega- 

 locephala und die Theorie der Chromosomenindividu- 

 alitat. Arch. Zellforsch., v. 3 (1/2): 181-268, figs. 

 1-7, pis. 7-11, figs. 1-51. 



1910a. — Ueber die Teilung centrifugierter Eier 

 von Ascaris megalocephala. Festchr. W. Roux. Arch. 

 Entwicldungsmech., v. 30 (2) : 101-125, figs. 1-32. 



1910b. — Die Potenzen der Ascan's-Blastomeren 

 bei abgeanderter Furchung. Festchr. R. Hertwigs. 

 v. 3: 131-214, figs. A-Y, pis. 11-16, figs. 1-39. 



225 



