118 THE STRUCTURE OF THE FOWL 



area, and is itself divisible into inner and outer 

 zones. 



Primitive Streak (Fig. 56). — During the first 

 day of incubation, an elongated, dim band, the 

 primitive streak, makes its appearance in the 

 pellucid area. The anterior end of the streak 



meso. 



Fig. 56. — Primitive Streak and its Surroundings. 



p.s., primitive strealc ; n.g., neural groove ; a. p., area pellucida ; 

 a.o., area opaca ; meso., mesoderm. 



as a rule is placed near the centre of the 

 pellucid area ; while the posterior end, some- 

 what broader, is less definite. It is now 

 generally accepted that the primitive streak 

 is produced by a linear thickening of the 

 ectoderm. 



As development proceeds the primitive 



