170 Proceedings of the Poyal Society 
of the dorsal blastopore, which has become distended by the increased 
bulk of the yolk developed in the ventral hypoblast of the original 
gastrula. The nerve cord in its original condition was a thickened 
line of epiblast surrounding the blastopore, and the formation of the 
medullary canal by which the nerve cord is removed from the 
surface of the body, is a process which origiually took place after 
the closing of the blastopore. The two processes are only accidentally 
connected with one another in actual development ; morphologically 
they are quite independent. 
Since the dorsal hypoblast in the Teleostean ovum is invaginated 
round the whole edge of the blastoderm, this edge forms the lip of 
the true primitive blastopore, and is homologous with the lip of the 
blastopore in the ancestral gastrula. As far as can be judged from the 
study of living pelagic ova, the posterior end of the blastopore is on 
the dorsal surface, and no part of it extends round the end of the 
body to the ventral side of the embryo. In the Amphibian ovum 
invagination of the dorsal hypoblast does not occur round the whole 
edge of the blastoderm simultaneously ; the invagination takes 
place first beneath the embryonic rudiment, and the rest of the lip of 
the blastopore is not inflected till a late stage when the yolk cells are 
almost completely enveloped by epiblast. But ultimately the whole 
edge of the blastoderm is inflected, and thus in the Amphibian, as 
in the Teleostean, the yolk blastopore or aperture by which the yolk 
protrudes, coincides with the true primitive blastopore. 
In the Elasmobranch ovum this is not the case. Only a propor- 
tionally small arc of the edge of the blastoderm is ever inflected, 
and the dorsal region of the embryo is entirely formed by con- 
crescence out of this part of the blastoderm. As far as the evidence 
goes at present, the inflected part of the blastodermic rim extends 
when it is folded together as far as the neurenteric canal, aud the 
aperture by which the yolk protrudes after the coalescence has 
reached this point is a hernia in the ventral wall of the body. 
Thus only the inflected arc of the edge of the blastoderm in the 
Elasmobranch is homologous with the whole lip of the blastopore 
in the ancestral gastrula, the rest of the edge corresponds to a 
rupture in the ventral wall of the body of the ancestor, or of the 
Teleostean embryo. 
In the Sauropsidan blastoderm, no part of the edge is ever 
