126 



FORMATION OF THE LAYERS. 



the posterior part of the area pellucida, and give rise to the opacity 

 of that part. 



At the edge of the area pellucida the hypoblast becomes con- 

 tinuous with a thickened rim of material, underlying the epiblast, and 

 derived from the orginal thickened edge of the blastoderm and the 

 subjacent yolk. It is mainly formed of yolk granules, 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. 132 and 133. 



The changes which next take place result in the complete differ- 

 entiation of the embryonic layers, a process which is intimately con- 



FlG. 93. DlAGKAMS ILLUSTRATING THE POSITION OF THE BLASTOPORE, AND THE RELATION 

 OF THE EMBRYO TO THE YOLK IN VARIOUS MEROBLASTIC VERTEBRATE OVA. 



A. Type of Frog. B. Elasmobranch type. C. Arnuiotic 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 blasto- 

 derm 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. 



nected 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 considered. The most striking peculiarity in the first 

 formation of the embryo Bird, as also in that of the embryos of all Am- 

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

 of the blastoderm, but are placed near its centre. Behind the embryo 

 there is however a peculiar structure the primitive streak above men- 



