394 



EMBRYOLOGY 



(Fig. 175). This region corresponds to the dorsal surface 

 of tlie larva. After the fusion of ectoderm and entoderm, 

 the lumen of the archenteron communicates with the 

 outer world, thus establishing the so-called dorsal pore 

 (Fig. 176). Johannes Muller, who, even in his time, was 

 acquainted with this process, considered the dorsal pore to 

 be the mouth of the larva ; but that is not its fate, for the 

 archenteron soon separates into two portions, of which the 

 one connected with the dorsal pore constitutes the funda- 



FiG. 174, 



Figs. 174 and 175. — Gastrula stages of Synapta digitat a (after Semsnea). In Fig. 175 

 the mesenchyma begins to develop. Bl, blastopore. 



ment of the water-vascular system and body cavity, while 

 the other becomes the intestine. With the multiplication 

 of its cells the latter acquires a knee-like bend (Fig. 177), 

 a,ud, while increasing in length, turns toward the ventral 

 side. Even before it reaches tliis, the communication be- 

 tween the upper and lower portions of the archenteron is 

 interrupted (Figs. 177 and 178). Of these two portions 



