236 



EMBRYOLOGY 



which presents an elongated fissure on one side. This con- 

 stitutes the blastopore of this peculiar gastrula. Also in the 

 forms observed by Hallez and in EJiabditis nigrovenosa the 

 blastopore exists in the form of a long slit (Fig. 110 B, II). 

 In the last-mentioned Nematode the gastrula arises by 

 the more active proliferation of the ectoderm cells, which 

 produce an epibolic overgrowth, embracing the ento-meso- 

 derm (Fig. 109 B, D), whereby a long slit, the blastopore, 

 persists on the ventral side (Fig. 110 B). Subsequently this 

 closes gradually from behind forwards. A transition between 



-mAO 



7ru«. 



Fig. 110. — A to E, different stages of development of RTiabdif is nigrovenos^fi (after 

 Goettk). hi, blastopore ; d, intestinal canal ; ent, entoderm ; g, fundament of the 

 genitalia; m, mouth; iii«s, mesoderm; n, fundament of the nervous system. 



gastrulation by invagination and by epiboly exists, according 

 to Hallez, in Oxijsoma. 



As regards certain facts of the subsequent embryonic 

 development which are not yet wholly clear, we have to 

 abide chiefly by the statements of Goette for Rhabditis 

 nigrovenosa. According to him, when the circumcrescence is 

 already far advanced, the formation of the mesoderm takes 

 place by the squeezing out of two cells from their connection 

 with the ento-mesoderm at the posterior end of the embryo 

 (Fig. 109 D). A comparison of these two cells with the 



