378 CHORDATA. 
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 primiteve streak. 
Fig. 272,.-VIEW OF THE AREA PELLUCIDA OF A CHICK’s 
BLASTODERM OF ABOUT 18 Hours. 
(After BALFourR.) 
Amniotic Head-fold. 
Neural Groove. 
Primitive Groove 
and Streak. 
Fig. 273.—ViEW OF CHICK’s BLASTODERM ABOUT 24 HOURS. 
(After Duvat.) 
Pro-amnion. Head. 
Area Opaca. 
Vitelline Vein. 
Mesoblastic 
Lateral Sheet. 
Protovertebre. 
Neural Groove. 
Primitive Groove 
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 avea opaca and the centre the area pellucida. These 
