Pseudalius minor do not have a blastocoele. Two layered 

 eel! plates such as occur in the latter instance were at 

 one time considered a type of gastrula, sterrogastrula, 

 but since this stage is followed by epiboly characteristic 

 of gastrulation it must be considered a type of blastula 

 for which the term placnla has been used. 



Gastrulation (Figs. 153 L-Q), on the other hand, is 

 the entrance of the entoderm and mesoderm into a hull 

 surrounded by ectoderm, this being completed at the 

 closure of the blastopore. Dependent upon the presence 

 and size or the absence of a blastocoele there are two 

 possible ways in which the ectoderm may come to sur- 

 round entoderm (Martini, 1908); (1) the cells may 

 retain their relative positions (synectic) or they may 

 not retain their relative positions (apolytic). In the 

 absence of a blastocoele the ectoderm may grow over the 

 entoderm either with or without change in cell positions 

 so we may have epibolic synectic or epibolic apolytic 

 gastrulation. Invagination, that is embolic gastrulation 

 is possible only if there is a blastocoele and in this case 

 it may be either apolytic or synectic- Epibolic apolytic 

 gastrulation is not known among nematodes but the 

 other possibilities are represented. Parascaris equorum 

 undergoes embolic apolytic gastrulation, Rliabdias and 

 Nematoxys embolic synectic, and Camallanus and Pseu- 

 dalius epibolic synectic (Fig. 153 P). 



Regarding the development of the mesoderm in nema- 

 todes there are certain points of interest. The meso- 

 dermal stem cells at the time of gastrulation are ar- 

 ranged in rows on either side of the entoderm as well 

 as anterior to it. They follow the entoderm in sinking 

 into the primary body cavity and later form two sub- 

 dorsal and two subventral strings on either side of the 

 entoderm. Since the individual cells maintain their 

 identity and no cavity is formed between them it would 

 not be proper to say that nematodes had a true coelome. 

 The individual cells would properly be termed a mesen- 

 chyme. However, certain mesodermal cells do cover the 

 organs in the body cavity and for that reason we may 

 say the body cavity is analogous but not homologous to 

 a coelome, i. e., a pseudocoelome. The mesoderm may 

 be said to have been derived from the entoderm since 

 it comes chiefly from a cell (S3) which also forms the 

 entoderm; but in Parascaris equorum as well as some 

 other nematodes part of the derivatives of the meso- 

 dermal stem cell (MSt) enter into the formation of the 

 ectoderm according to some authors. Strictly speaking 

 it would seem preferable to consider six germ layers, the 

 ectoderm, mesoderm (somatic musculature and isolation 

 tissue), entoderm, esophagus (St-Sl), somatic part of 

 gonad (S5) and germinal layer (P5). 



The SI cell group in nematodes forms the greater part 

 of the epithelium which is usually so arranged that the 

 nuclei and the cell bodies of the cells are situated in the 

 dorsal, ventral, and lateral chords. It also contributes 

 to the formation of the esophagus, the nervous system, 

 and the excretory system. 



The S2 cell group forms the greater part of the meso- 

 derm, that is the longitudinal muscles, transverse muscles, 

 and probably the isolation tissue, as well as the muscu- 

 lature of the esophagus; it also forms the mesenteron 

 or intestine. 



The S3 cell group forms the ectodermal epithelium of 

 the posterior part of the body and contributes to the 

 formation of the nervous system and the musculature 

 of that part of the body. 



The S/f cell group is known to form the rectum and 

 rectal glands and may also form some muscular tissue. 



The S5 cell group according to Pai (1928) forms the 

 outer covering of the gonads and the epithelium of the 



Fig. 152. 



A-0 — Turbatrix aceti (A — Fertilized ovum; B — 2 cell stage; C — 

 Beginning second cleavage ; D — 4 cell stage ; E — 6 cell ; F — 16 

 cell; G-H — 24 cell; I-J — 141 cell: K — 26 cell; L — 82 cell; M— 

 171 cell; X — Early definitive embryo; O — Tadpole stage) P-DD — 

 Camallanus lacustris (P — 2 cell stage; Q — 4 cell; R — 2S cell, 

 dorsal view ; S — 28 cell, ventral view ; T — 177 cell, dorsal view ; 

 U — 177 cell, ventral view ; V — 354 cell, ventral view : W-X — 

 Cross sections of embryo slightly older than V, W — anterior region. 

 X — Posterior region ; Y-AA — Cross sections of anterior, mid and 

 posterior regions of still older embryo : BB-CC — Sagittal and 

 surface views of early definitive embryo ; DD — Anal region of 

 mature larva) : EE-HH — Rhabdias bufonis ( EE-FF — Surface and 

 sagittal views of early definitive embryo ; GG — Surface view of 

 tadpole stage: HH — Cross section of stage shown in EE and FF). 



A-O. After Pai. 192S, Ztschr. Wiss. Zool.. v. 131 (2) ; P-AA, 

 After Martini. 1903. Idem., v. 74 (4) ; BB-DD, After Martini. 

 1906, Idem., v. 81 (4) ; EE-HH, After Martini, 1907, v. 86 (1). 



gonoducts. If nematodes can be said to have the 

 homologue of a true coelome it would be the lumen of the 

 gonoducts since it is formed as a cavity between cells, 

 but the positional relationship of the S5 group with the 

 entoderm makes its consideration as mesoderm rather 

 questionable. 



At the time of hatching from the egg, nematodes are 

 fully formed, the tissues differentiated and functional 

 with the exception of the reproductive system. In some 

 nematodes no further division of cells takes place except 

 in the gonads and structures either directly or indirectly 

 connected with reproduction. In all instances known 

 the chords are cellular rather than syncytial, there 

 being five rows of cells in the anterior part of the 

 body of most nematodes studied (a dorsal, two lateral, and 

 two ventral) while in the remainder of the body there 

 are two dorsolateral, two lateral, two ventrolateral, and 

 two ventral, the dorsal being absent but there is a 

 thickening of the hypodermis in the dorsal region. 

 Changes from this condition take place during later 

 development or not at all. 



The somatic musculature is platymyarian and mero- 

 myarian in the newly hatched larva but it may become 

 coelomyarian and polymyarian later. In such an in- 

 stance we have to recognize the division of functional 

 muscle cells, certainly highly diffei-entiated. 



The esophagus of the larva is similar to that of the 

 adult in some forms; whether or not multiplication of 

 cells may later take place is unknown. 



The intestine of the larva is composed usually of 2 

 rows of cells, the lumen being formed by a separation 

 of parallel rows of cells rather than from the archen- 

 teron. This formation of the lumen seems to be of no 

 general significance since it is the only means by which 

 a lumen could develop in forms with so little blastocoele. 



The nervous system, at least in some forms, appears 

 to be of the same number of cells in the larva as in 

 the adult, with the exception of cells innervating genital 

 papillae. 



The excretory system of the larva is the one system 

 of which our knowledge is entirely inadequate. It is 

 usually stated to be formed by a single cell of the 

 ectoderm (SI) but its development has not been satis- 

 factorily traced. 



Regarding the embryology of particular nematodes, we 

 find that thus far no member of the Aphasmidia has 

 been studied though many members of the Phasmidia 

 have. These belong to several diverse groups, Rhabdias 

 bufonis, Rhabditis terricola (partially), Diplogaster long- 

 icauda (partially), and Turbatrix aceti among the Rhab- 

 ditina; Metastrongylus clongatus, Pseudalius inflexus, 

 and P. minor among the Strongylina; Ascaris lumbricoides 

 (partially), Toxocara coin's (partially), Parascaris equor- 

 um, Nematoxys ornatus, and Syphaeia obvelata among 

 the Ascaridina; and Camallanus lacustris of the Camal- 

 lanina. Of these forms it appears best to limit our 

 descriptions to Turbatrix aceti, Rhabdias bufonis, Paras- 

 caris equorum, and Camallanus lacustris, comparing other 

 forms with them whenever it appears advisable. 



Turbatrix aceti (Fig. 152). Pai (1927) worked out 

 the development of this species in a very complete man- 

 ner. He found that the end of the ovum at which the 

 sperm entered is destined to form the anterior end of 

 the embryo. From the first cleavage which is trans- 

 verse, two very slightly unequal cells result, a larger 

 anterior one designated as SI, and a smaller posterior 

 one, PI. By the second cleavage SI, dividing somewhat 

 horizontally and obliquely, gives rise to a dorsal cell B 

 and an anterior cell A. Division of PI follows shortly, a 

 ventral cell S2 and a posterior cell, P2 resulting. The 

 second somatic stem cell, S2, is said to form the esopha- 

 gus, the intestine or mesenteron, and the mesoderm, and 

 for that reason may be designated EMSt, i. e., entoderm, 

 mesoderm, stomodeal stem cell. At the third cleavage the 

 Si cells, A and B, divide longitudinally giving the em- 

 bryo a distinct bilateral symmetry which it retains 

 throughout the remainder of its development. This 

 is followed by a transverse division of the ventral cell, 

 S2, forming an anterior ventral cell MSt and a posterior 

 ventral cell E. Thus in the seven cell stage there are 

 four dorsal ectodermal cells, two ventral ento-mesodermal 

 cells derived from the second somatic stem cell, and a 

 single large posterior undifferentiated cell, P2. In later 

 divisions the blastomeres procede at an even more un- 

 equal rate, there being a distinct tendency for the S2 



219 



