REPRODUCTION AND DEVELOPMENT 273 



lip of the blastopore becomes differentiated into a plate that sinks 

 downward to form a groove. This plate of ectodermal cells is 

 destined to form practically the entire nervous system (Fig. 149). 

 The boundaries of the groove close over and fuse, leaving the future 

 nerve elements as a tube, under the ectoderm and surrounded by 

 mesoderm. The dilations of the anterior end of this neural tube to 



THYROID 



VISCERAL POUCHES 



TRACHEA 

 STOMACH 



PANCREAS 



INTESTINE 



-MOUTH 



LIVER 



YOLK STALK 



ALLANTOIS 

 ANAL PLATE 



Fig. 185. — The primitive digestive system of the chick embryo after 4 days' in- 

 cubation of the ^gg. (After Patten: Embryology of the Chicly, published by P. Blakis- 

 ton's Son and Company.) 



form the basic portions of the brain have already been described 

 (p. 217), as have also the origin of the ganglia of the spinal cord 

 and of the sympathetic nervous system, and other parts of the 

 adult vertebrate nervous mechanism. 



The Alimentary Canal. The endoderm in invertebrates be- 

 comes rapidly differentiated into a functional alimentary tract. In 

 diploblastic forms and in the more primitive triploblastic animals, 

 for example the Platyhelminthes, it does not acquire a second open- 

 ing. But in the more advanced invertebrates and in all chordates 

 a contact forms between the ectoderm and the anterior end of the 



