COMPARISON OF THE GERMINAL LAYERS. 287 



case. The medullary plate, instead of lying at the edge of the 

 blastoderm, lies in the centre, and its formation is preceded by 

 that of a peculiar structure, the primitive streak, which, on the 



FIG. 175. 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 the yolk not yet enclosed by 

 the blastoderm. 



formation of the medullary plate, is found to lie at the hinder 

 end of the latter and to connect it with the edge of the blasto- 

 derm. 



The possibility of a comparison between the Sauropsida and 

 the Elasmobranchii depends upon the explanation being possible 

 of (i) the position of the embryo near the centre of the blasto- 

 derm, and (2) the nature of the primitive streak. 



The answers to these two questions are, according to my view, 

 intimately bound together. 



