NEMATHELMINTHES 



235 



egg is divided into an ectodermal and an ento-mesodermal half. In 

 Rhahditis nigrovenosa, according to Goette, the ventral and dorsal sides 

 and the anterior and posterior ends of the embryo can be recognized even 

 at this time. The ento-mesoderm divides first into two blastomeres. 

 The ectodermal blastomere sends out a process dorsally over both of 

 these (Fig. 109 A), and a newly formed ectodermal sphere is then situated 

 at this point. In the further division 

 of the ectoderm and ento-mesoderm 

 the elements of the former push them- 

 selves more and more over those of the 

 latter, and thus as a whole come to lie 

 more dorsally (Fig. 109 B). In subse- 

 quent stages two cells lying close together 

 at the former ectodermal pole of the egg 

 indicate the tail-end of the embryo (Fig. 

 110 A, B), while the head-end lies oppo- 

 site. 



Whereas Goette makes the separation 

 of the mesoderm from the entoderm take 

 place later, it occurs, according to Hallez 

 (in Ascaris and Rhahditis aceti), even in 

 the eight-cell stage, in which two meso- 

 derm cells are constricted off from two 

 entoderm cells. In the twenty-four-cell 

 stage, the blastula, with a small cleavage 

 cavity, is formed, the dorsal part of which 

 is composed of the ectodermal cells, the 

 ventral of the entodermal and meso- 

 dermal cells. 



e-jiA. 



Fig. 109.—^ to D, cleavage 

 stages and formation of the 

 germ-layers in Rhahditis nigra- 

 venosa (after Gobtte). ect, ec- 

 toderm ; ent, entoderm ; mes, 

 mesoderm. 



Gastrulation takes place in vari- 

 ous ways, according to the form of 

 the blastula. In Ascaris tnegalo- 

 cephala an invagination gastrula is 

 formed, the archenteron of which 

 is very shallow, owing to the 

 shape of the thick-walled blastula 

 (Hallez). The process of gastrulation in Ciicullanus elegans 

 takes place in a peculiar manner, as was demonstrated by 

 BÜTSCHLi. In this form the blastula stage consists, as has 

 been mentioned, of a bilaminar cell-plate. This shape is 

 soon lost, however, for the cells of one layer multiply more 

 rapidly than those of the other, and therefore a bending 

 toward the latter ensues. Finally a kind of tube is formed, 



