152 



FORMATION OF THE LAYERS. 



with a varying number of cells and nuclei imbedded in it. It is 

 known as the germinal wall, and is spoken of more in detail on 

 pp. 160 and 161. 



The changes which next take place result in the complete 

 differentiation of the embryonic layers, a process which is 



FIG. 93. DIAGRAMS ILLUSTRATING THE POSITION OF THE BLASTOPORE, AND 

 THE RELATION OF THE EMBRYO TO THE YOLK IN VARIOUS MEROBLASTIC VERTE- 

 BRATE OVA. 



A. Type of Frog. B. Elasmobranch type. C. Amniotic Vertebrate. 

 mg. medullary plate ; ne. neurenteric canal ; bl. portion of blastopore adjoining the 

 neurenteric canal. In B this part of the blastopore is formed by the edges of the 

 blastoderm meeting and forming a linear streak behind the embryo ; and in C it forms 

 the structure known as the primitive streak, yk. part of yolk not yet enclosed by the 

 blastoderm. 



intimately connected with the formation of the structure known 

 as the primitive streak. The meaning of the latter structure, 

 and its relation to the embryo, can only be understood by 

 comparison with the development of the forms already con- 

 sidered. The most striking peculiarity in the first formation of 

 the embryo Bird, as also in that of the embryos of all Amniota, 

 consists in the fact that they do not occupy a position at the edge 



