72 



THE DEVELOPMENT OF THE CHICK 



a.p. 



Jfes. 



Ent. TJ). 

 ■prp 



Ent.Th. 



-pr.gr. 

 -pr.f. 



s.^r 



Fig. 36. — A. Intermediate stage of the formation 

 of the primitive streak of the sparrow. (After 

 Schaninsland.) 



B. Fully formed primitive streak of the spar- 

 row. (After Schaninsland.) 



a. o., Area opaca. a. p., Area pellucida. Ent. 

 Th., Thickenino; of entoderm. Mes., Mesoderm, 

 pr. f., Primitive fold. pr. gr., Primitive groove. 

 pr. p., Primitive pit. pr. str., Primitive streak, 

 s. gr., Sickle groove. 



At first the surface of the primitive streak is even, but, as 



it elongates, a groove appears down its center. This groove is 



I known as the primitive groove ; it is bounded by the primitive 



\folds and terminates abruptly in front in a pit, the primitiv^e 



(pit, which corresponds to the neurenteric canal of other verte- 



