submedian regions. In the anterior part of the body 

 this causes the / and L cell groups to be incompletely 

 separated from the d cell row dorsally and the g and G 

 Cell rows ventrally. The result is that there are four 

 chords, the dorsal and two lateral chords consisting of 

 one cell row each, and the ventral chord of two cell rows. 

 This takes place in the section of the embryo covered 

 by d 1.1-20, I and L 8-10, g and G $-10. In the remainder 

 of the embryo the primordia of the muscles press and 

 separate the alternate cells of the dorsal cell row and the 

 ce.ls of the two ventral cell rows. This causes the form- 

 ation of two lateral rows of three cells each, the dorso- 

 lateral cell rows being formed by d and d cells, the 

 ventrolateral by g and G cells, the result being two 

 lateral chords of three cell rows. There remains a 

 thickening of the mediodorsal part of the epidermis 

 which is free of nuclei, the dorsal chord, and a ventral 

 thickening- which contains the small Si and S3 cells form- 

 ing the ventral chord with its ganglia. 



The mesoderm giving rise to the subdorsal and sub- 

 ventral muscle bands is derived chiefly from cells of 

 the ;!/ group but posterior cells of the St group also 

 contribute. As they push out between the covering 

 epithelial cells they become completely differentiated, and 

 form overlapping double rows of platymyarian muscles 

 in each sector. 



Immediately after the closure of the, ventral groove we 

 find a nearly solid mass of cells anterior to the in- 

 testine. This is the primordium of the esophagus (Fig. 

 l. r >2 BB). It has apparently arisen from two cell groups, 

 the Sf and small cells of the SI group. The lumen of 

 the esophagus has in its origin no connection whatever 

 with the ventral groove. The cells enter the body cavity 

 as a mass, becoming arranged in a triradiate pattern. 

 The lumen is formed by separation of the cells. Already 

 in the stage represented above, the various cells may 

 be recognized which are later present in the adult. The 

 nuclei are very closely placed behind one another, there 

 being a total of 66. 



As the embryo becomes more elongated the nuclei are 

 separated and at hatching come to form an esophagus 

 consisting of an anterior part containing two groups 

 of three marginal nuclei, two groups of six radial nuclei 

 (Fig. 153 A) ; and a posterior part containing two groups 

 of three marginal nuclei, two groups of six radial nuclei, 

 two groups of two subventral radial nuclei, six groups of 

 three radial nuclei (Fig. 153 A). The same number of 

 nuclei was observed in the adult stage. Part of the 

 radial nuclei are probably nuclei of the esophageal glands 

 and part nuclei of the esophago-sympathetic nervous 

 system. Martini considers that the gland cells, marginal 

 cells, and nerve cells of the esophagus are derived from 

 small cells of the Si cell group while the radial muscle 

 cells are derived from the St cell group. 



The esophago-intestinal valve is formed from the same 

 general tissues as the esophagus. In the early postgas- 

 trular stage (Fig. 152 BB) five nuclei may be seen 

 between the esophagus and the intestine; at hatching 

 (Fig. 153 A) these five nuclei comprise a large dorsal 

 nucleus, two subdorsal, one left lateral, and one ventral. 



Fig. 153. 



A.-CC — Camallanus lacustris (A — .Mature larva, showing various 

 nuclei ; B — Slightly younger larva showing digestive tract ; !■'. 

 flat nuclei, i. e.. between esophageal radii : K, corner nuclei, 

 i. e., opposite esophageal radii ; E. Intestinal nuclei ; d, subdorsal 

 lambda & 1. lateral hypodermal nuclei. g & gamma 

 subventral hypodermal nuclei ; Ag, last cell pair of rectum. C — 

 Cross section of larva at stage shown in A) ; D-l and R — Rhaodias 

 lufonis (D — Intestinal region of larva; E-F — Tangential and 

 sagittal sections of embryo with wide open blastopore ; G-H and 

 R — Tadpole stage. G & R cross sections of anterior and posterior 

 regions, H. sagittal ; I — Cross section of nearly mature larva 

 showing intestine and primordial germ cell); J-K — Para 

 equorum (Sagittal and tangential views of 102-202 cell stage) : 

 L-Q — Methods of gastrulation (jL — Coelobiastula : M — Epibolic- 

 synectic gastrulation as in Ascaris and Parascaris ; N — Epiholic- 

 apolytic gastrulation, unknown in Nematoda ; O — Placula or sterro- 

 blastula : P — Epibolic-synectic gastrulation as in Camallanus and 

 Pseudalius ; Q — Embolic-synectic gastrulation as in Rhaodias and 

 Nematoxys : Those above the horizontal line are embolic, those 

 below epibolic while those on the readers' left are apolytic and 

 on the right synectic). 



A-C, After Martini. 1906. Ztschr. Wiss. Zoo]., v. 81 (4) : & I, 

 After Martini, 1907, Idem. v. Sfi (1) : L-Q. After Martini. 1908, 

 Idem. v. 91 (2) : EG & R, After Neuhaus. 1903. Jena Ztschr. 

 Naturw.. v. 37, n. F. v, 30 (4) ; J-K. After H. Mueller. 1903, 

 Zoologica (41). 



The intestine in the early postgastrular stage (Fig. 

 152 BB) consists of 2 lateral rows of 8 large 

 cells derived from E. With elongation there is a slight 

 torsion of these cells and the two rows separate in the 

 middle forming an irregular zigzag lumen surrounded 

 by a dorsal and a ventral cell row. 



The rectum, in so far as known, is derived from the 

 S 1 cell group, this group being enth'ely enclosed at 

 gastrulation. The proctodeum is formed (Fig. 152 DD) 

 through the separation of cells in this region. A group 

 'if 11 small cells lies between the posterior end of the 

 intestine and the ventral side of the body. As in the 

 case of the esophagus the nuclei later separate through 

 elongation of the organism. Four cells surround the 

 proctodeum at its junction with the body wall (AGl and 

 .1 (/.') two being dorsal and two ventral; two lateral cells 

 are anterior to these (Tg) ; a group of three large cells 

 one dorsal and two ventral (Dg) lies anterior to these; and 

 there are two additional cells, one dorsal and one ventral, 

 connecting the intestine and rectum. No increase in 

 number of cells takes place in later development. 



Soon after the completion of gastrulation the genital 

 primordium is recognizable as four cells, two of which 

 (the terminal cells) cover the other two (the primordial 

 germ cells). Martini considers the terminal cells as 

 probably originating- from the M cell group. It seems 

 more probable, in the light of Pai's observations on 

 Turbatrix aceti (See p. 220), that the anterior cell 

 resulting from the fifth cleavage in the P cell line (so 

 called Pi I or S5) formed this layer. In case Pai is 

 correct, the two primordial germ cells present at hatch- 

 ing resulted from the sixth cleavage of the P stem cell. 



Regarding the development of the nervous system 

 little is known except that it may form the small cells 

 of the SI and S3 cell groups. Nothing whatsoever is 

 known regarding the origin of the excretory system. 



Parascaris equorum. (Fig. 154). The embryology of 

 the horse ascarid usually called Ascaris megalocephala, 

 has been worked on by Boveri (1892, 1899, 1909, 1910 a, 

 b), zur Strassen (1896, 1899 a, b), Miiller (1903), Zoja 

 (1896), Bonfig, (1925), Girgoloff (1911), Hogue (1911), 

 Schleip (1924) and Stevens (1925) and in most of the 

 results there is entire agreement. The lineage has been 

 followed up to the 802 cell stage by Miiller at which 

 stage the embryo is completely developed and somewhat 

 elongate, but has not reached the first larval stage. The 

 large number of cells and the difficulty of following 

 postembryonic stages, due to the life history of the 

 species, makes it impractical to follow the differentiation 

 of particular tissues. 



In the general embryology Parascaris equorum is nearly 

 identical with Turbatrix aceti but Boveri's beautifully 

 illustrated work (1899) shows that chromatin material is 

 lost from the nuclei during the division of somatic stem 

 cells, a fact which indicates a very definite cytological 

 basis for the unequal potentiality of the embryonic 

 blastomeres. Chromatin diminution is not known in 

 other groups of nemas though the same differentiations 

 in potentialities are present. 



The cleavage pattern of Parascaris equorum (Fig. 154) 

 is identical with that of the previously described species. 

 At the 56 cell stage (sixth cleavage) the cells are as 

 follows: 32 of the SI group, 4 St, 4 M, 4 E, 8 C, 2 D and 

 2 P. Thereafter all of the cells except the C and P 

 lines divide (seventh cleavage) forming a 102 cell stage 

 at which time there is a well formed gastrula (Fig. 

 154 AA-EE) the anterior lip of which is bordered by 8 

 stomodeal cells (St) while the posterior lip is bordered by 

 4 proctodeal cells (D). During this division some of the 

 SI cells divide unequally and to those which have been 

 more carefully traced in subsequent divisions Mueller 

 (1903) gave a simplified terminology, g and G correspond- 

 ing to pairs of cells as the fifth cleavage (such as A IV) 

 ■"further divisions forming ga, gb, then gar, gal, gbr, 

 and gbl; others were similarly renamed oyr, uyr, oyl, uyl, 

 xrl, kbr, etc. These cells contribute a large part of the 



••The student who desires to trace individual cell lines will 

 find a very complicated terminology, especially since zur Strassen 

 used the system I for the first stem cell, A for the first divison 

 thereof, r for right. I for left etc.. so that larlBoy corresponds 

 to gar in the new nomenclature. In his later work. Martini used 

 a simplification which involved some of the same letters as thoso 

 used by Mueller but for different cells. See Camallanus lacustris. 



223 



