•258 EMBRYOLOGY 



the forward growth of the ectoderm, the mesoderm cells 

 are now forced inward, and the invagination of ectoderm 

 cells, which succeeds it (Fig. 115 D, E), subsequently pro- 

 duces the trochal apparatus and the oesophagus. 



The outer form of the embryo is now changed in such a 

 way that an anterior, posterior, and two lateral protuberances 

 can be distinguished (Salensky, Tessin). On the surface 

 which bears the blastopore, these regions of the body are 

 seen to be separated from one another by shallow grooves 

 (Fig. 115 F). The posterior elevation bends forward, and 

 growing further in the same direction, forms the foot (or 

 caudal appendage) of the Rotifer. 



TpssiN seeks to refer the anterior and lateral elevations (cephalic and 

 lateral lobes) to the lobular processes of the Turbellaria, especially to 

 those of the larva of Stylochus. Inasmuch as the Eotatoria do not pass 

 through any real laryal stage, the lobular processes would have become 

 rudimentary. In the further course of development the cephalic and 

 lateral processes are again smoothed out, and can no longer be recog- 

 nized as special structures. 



Concerning the origin of the inner organs even Tessin can give little 

 definite information. We have already mentioned that he derives the 

 trochal organ and the most anterior part of the intestinal canal from an 

 ectodermal invagination. On the other hand, he combats the discoveries 

 of Salensky, for he derives the masticating stomach (pharynx), which 

 is provided with jaws, fi'om the entoderm ; Salensky maintains that this 

 part is of ectodermal origin. According to Tessin, by far the largest 

 part of the intestine (together with the appended glands) arises from the 

 entoderm, for the latter extends far backwards ; it is said even to send a 

 process into the caudal appendage. The hind-gut arises by means of an 

 invagination of the ectoderm (Salensky, Joliet). 



The further fate of the mesodermal fundament remained obscure to 

 Tessin. The statements regarding the origin of the nervous system and 

 the genital organs are of too doubtful a nature for us to consider them. 

 Nothing is as yet known concerning the formation of the excretory 

 organs. 



The development of the male of BracMonus urceolaris, 

 which, as is known, is very simply constructed, takes place, 

 according to Salensky, in the same way as that of the 

 female. The degenerative process^ which characterize tlie 



