378 CHORD ATA. 



because, as in the lip of the blastopore [cf. Amphioxus and Frog), the 

 three layers are there continuous. This thickening gives rise to an 

 opacity called the primitive streak. 



Fig. 272. — View of the Area Pellucida of a Chick's 

 Blastoderm of about 18 Hours. 



(After Balfour.) 



Amniotic Head-fold. 



Neural Groove.- 



Primitive Groove- 

 and Streak. 



Fig. 273. — View of Chick's Blastoderm about 24 hours. 



(After Duval.) 

 Pro-amnion. ^^mmmmmm^-... Head. 



Area Opaca. 



Mesoblastic 

 Lateral Sheet. 



Neural Groove. 



Vitelline Vein. 



Protovertebrae. 



Primitive Groove r^"' 

 and Streak. 



In a similar manner the whole rim of the blastoderm has a thicker 

 layer of cells than the middle and gives rise to an opacity. Hence the 

 rim is called the area opaca and the centre the area pellucida. These 



