380 CHORDATA. 
The mesoblast is differentiated from the hypoblast as paired sheets 
of cells which grow from the primitive streak forwards in two wings. 
They give rise dorsally to protovertebre along the sides .of the neural 
tube, and ventrally they slowly follow in the track of the epiblastic rim 
round the yolk. In the median dorsal line a rod of hypoblast cells 
forms the notochord. 
Fig. 276.—D1acram OF DEVELOPING CHICK. 
Amniotic Fold. 
Embryo. 
Notochord. 
Mesoderm. 
Rim of Rim of 
Blastoderm. Yolk. Blastoderm. 
Amnion. Amniotic Cavity. 
Allantois. 
Wall of Yolk-sac. 
A, The blastoderm is gbout 2-sths round the yolk. _B, It is about 3-4ths 
round. C, Phe blastoderm has nearly enveloped the yolk. 
For later stage see E on page 428. 
At the end of the first day the embryo has about half-a-dozen proto- 
vertebrze, an open neural tube, a blastoderm extending about #-inch in 
diameter and mesoblast growing out under the epiblast. In front of 
the anterior end the mesoblast sheets do not meet till late, hence here 
the blastoderm is only two-layered. This area is sometimes called the 
