I04 



ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



the gastrula (Fig. 44, B). The characteristic features of the gastrula 

 are : (i) that its shape is cup-like, (2) that it has a simple internal cavity 



— the archenteron — which communicates with the exterior by a single 

 opening — the primitive mouth or protostoma, and (3) that its wall consists 



