654 EARLY DEVELOPMENT OF THE LACERTILIA. 



rudiment of the continuity of these layers at the dorsal lip of 

 the blastopore in Elasmobranchii, Amphibia, &c. The second 

 section of the blastopore in Elasmobranchii or yolk blastopore 

 is, I believe, partly represented by the primitive streak. The 

 yolk blastopore in Elasmobranchii is the part of the blastopore 

 belonging to the yolk sac as opposed to that belonging to the 

 embryo, and it is clear that the primitive streak cannot cor- 

 respond to the whole of this, since the primitive streak is far 

 removed from the edge of the blastoderm long before the yolk 

 is completely enclosed. Leaving this out of consideration the 

 primitive streak, in order that the above comparison may hold 

 good, should satisfy the following conditions : 



1. It should connect the embryo with the edge of the blasto- 

 derm. 



2. It should be constituted as if formed of the fused edges of 

 the blastoderm. 



3. The epiblast of it should eventually not form part of the 

 medullary plate of the embryo, but be folded over on to the 

 ventral side. 



The first of these conditions is only partially fulfilled, but, 

 considering the rudimentary condition of the whole structure, no 

 great stress can, it seems to me, be laid on this fact. 



The second condition seems to me very completely satisfied. 

 Where the two edges of the blastoderm become united we should 

 expect to find a complete fusion of the layers such as takes 

 place in the primitive streak ; and the fact that in the primitive 

 streak the hypoblast does not so distinctly coalesce with the me- 

 soblast as the mesoblast with the epiblast cannot be urged as a 

 serious argument against me. 



The growth outwards of the mesoblast from the axis of the 

 primitive streak is probably a remnant of the invagination of the 

 hypoblast and mesoblast from the lip of the blastopore in Am- 

 phibia, &c. 



The groove in the primitive streak may with great plausi- 

 bility be regarded as the indication of a depression which would 

 naturally be found along the line where the thickened edges of 

 the blastoderm became united. 



With reference to the third condition, I will make the follow- 

 ing observations. The neurenteric canal, as it is placed at the 



