72 



THE DEVELOPMENT OF THE CHICK 



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

 of the primitive streak of the sparrow. (After 

 Schauinsland.) 



B. Fully formed primitive streak of the spar- 

 row. (After Schauinsland.) 



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

 Th., Thickening 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 

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

 folds and terminates abruptly in front in a pit, the primitive 

 pit. which corresponds to the neurenteric canal of other verte- 



