38o 



CHORD AT A. 



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 protovertebi^cE 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. — Diagram of Developing Chick. 



Notochord. 



Amniotic Fold. 



^ Embryo. 



Mesoderm. 



Rim of 

 Blasto- 

 derm. 



Yolk. 



Rim of 



Blastoderm. 



,, Rim of 



Yolk. Blastoderm. 



Amnion. Amniotic Cavity. 

 .Serosa. 



Allantois. 



Yolk. 



Wall of Yolk-sac. 



A, The blastoderm is about 2-5ths round the yolk. B, It is about 3-4ths 



round. C, The 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- 

 vertebrae, an open neural tube, a blastoderm extending about f-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 



