46 Vertebrate Embryology 



The anterior end of the digestive tract early 

 becomes expanded into what may be called 

 the pharynx, and there is a similar though 

 smaller expansion at the posterior end (Figs. 

 12 TP and 22). The entoblast at first forms 

 a more distinct layer on the dorsal side of the 

 mesenteron than it does on the ventral side, 

 but it very soon extends entirely around the 

 cavity as a distinct layer. 



In the frog the primitive mouth or blastopore 

 closes entirely, so that the digestive tract may 

 be for a short time a completely closed cavity, 

 but in some other animals the blastopore per- 

 sists as the permanent anus. 



Shortly before hatching, a depression of the 

 ectoblast may be seen on the ventral side of 

 the head (Fig. 14, DS} ; this is the beginning 

 of the stomodaeum. The depression becomes 

 deeper and deeper until it is separated from 

 the front of the pharynx by only a thin septum 

 (Fig. 17, S D). Soon after hatching this sep- 

 tum becomes perforated and the mouth open- 

 ing is formed. The lips now grow forward, 

 the jaws become formed, and the tadpole 

 begins to take food from the surrounding 

 water (Fig. 18). 



The proctodaeum or anal opening is formed 



