15 



resemblanco witli stages of devolopment of tliis class of animals. 

 ScLATEK ') also lias pointed out siicli analogios. 



In the later course of devolopment the American species follow 

 their own way. The new entoderm arises as a compact mass of 

 cells at the spot, where the virtual blastoporus is to be sought 

 for. It however grows in a direction, contrary to that in which 

 the embryonic entoderm is delaminated and fills up the amniotic 

 cavity. According to this explanation an inversion of the em- 

 bryonic layers takes place (see fig. C). This has already been pointed 

 out by ScLATER, who however did not give an explanation of 

 this phenomenon. 



1) 1. c, p. 358. 



