72 THE DEVELOPMENT OF THE CHICK 
Fia. 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 opaca. 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- 
