RELATIONS OF YOLK TO GASTRULA 1N TELEOSTEANS. 27 , 
The yolk blastopore in Petromyzon coincides with the 
primitive blastopore, but the investigation of this form has not 
been complete enough to throw much light on the history of 
the posterior end of the blastopore; the neurochord is at first 
solid, as in Teleosteans, and, so far as is known, the whole of the 
blastopore belongs to the dorsal surface. 
In Acipenser the edge of the blastoderm is inflected for its 
whole circumference, and the primitive blastopore is not 
modified any more than in the types already considered. As 
far as is known the blastopore at its posterior end is entirely 
enveloped by the medullary folds. The yolk is contained 
within the walls of the intestine, but this can easily be 
explained ; the ventral wall of the intestine is formed from the 
lower part of the nucleated yolk instead of, as in Amphibia, 
from the upper part. The history of the invagination process 
in Lepidosteus is unknown. 
In all the above types, with the doubtful exception of Am- 
phibia, sooner or later the whole of the edge of the blastoderm 
is inflected, the yolk blastopore coincides with the ancestral 
blastopore. In Elasmobranchs this is not so; a great part of 
the edge of the blastoderm is never inflected. From what is 
known of the development of this type it seems to me perfectly 
clear that the dorsal wall of the embryo is formed as in the 
preceding types by the coalescence of the two halves of the 
inflected edge of the blastoderm on either side of the embryonic 
rudiment. But the coalescence of the inflected rim of the 
blastoderm in Elasmobranchs is not the same thing as the 
envelopment of the yolk by the blastoderm. A great part of 
the yolk remains uncovered after the dorsal side of the embryo 
and the neurenteric canal are completely formed. But it is 
the uninflected part of the blastodermic edge which coalesces 
after the period just mentioned. The only way of explaining 
the relation of the Elasmobranch development to that of 
Teleosteans and Amphibia, as it seems to me, is the following: 
—The inflected are of the blastoderm edge in the former is 
homologous with the whole of the edge of the blastoderm in 
the latter, and that are alone represents the primitive ancestral 
