COMPARISON OF THE GERMINAL LAY E US. 239 



peripheral position, as well as the true neurenteric part of the Elas- 1 

 mobranch blastopore. 



Fid. 175. Diagrams ili-ustbating the position of the blastopore, and the relation 

 OF the embryo to the yolk in various mekoblastic Vertebrate ova. 

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

 mg. medullary plate; iie. neurenteric canal; hi. portion of blastopore adjoining the 

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

 toderm 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. 



This view of the nature of the primitive streak, which is 

 diagrammatically illustrated in fig. 175, will be rendered more clear 

 by a brief review of the early developmental processes in the 

 Sauropsida. 



After segmentation the blastoderm becomes divided, as in Elasmo- 

 branchii, into two layers. It is doubtful whether there is any true 

 representative of the segmentation cavity. The first structure to 

 appear in the blastoderm is a linear streak placed at the hind end of 

 the blastoderm, known as the primitive streak (figs. 175 C, hi and 176 

 pr). At the front end of the primitive streak the epiblast and hypo- 

 blast become continuous, just as they do at the dorsal lip of the 

 blastopore in Elasmobranchii. Continued back from this point is a 

 streak of fused mesoblast and epiblast to the under side of which a 

 linear thin layer of hypoblast is more or less definitely attached. 



A further structure, best developed in the Lacertilia, appears in 



