28 J. T. CUNNINGHAM. 
blastopore. The rest of the edge of the blastoderm in Elasmo- 
branchs is really a hernia caused by the greater size of the yolk. 
The homologous parts in the two cases are represented in the 
two diagrams I and II, Pl. 1V. The uninflected part of the 
blastoderm edge in the Elasmobranch, c in Diagram II, cor- 
responds with a rupture of the blastoderm in the Teleostean, 
extending to its edge (x), and represented by the dotted line 
(c) in Diagram I. 
An important question here arises, suggested by the uncer- 
tainty before pointed out, concerning the blastopore in Triton. 
To what part of the embryo does the inflected arc in Elasmo- ~ 
branchs extend? To judge from Balfour’s results we should 
conclude that the neurenteric canal in this type marks the end 
of the inflection or invagination, and we should be able to 
decide that the closing of the ancestral blastopore is repre- 
sented by the coalescence as far as the neurenteric canal and 
no farther. The results of Sedgwick and Miss Johnson, if 
correct, must be interpreted in one of two ways. Either the 
neurenteric canal is in all Vertebrates the termination of the 
ancestral blastopore, in which case the ventral opening in 
Triton is simply a hernia corresponding to the yolk hernia in 
Elasmobranchs, and having no inflection at its edge; or the 
ancestral blastopore extends beyond the neurenteric canal. If 
the latter interpretation prove to be true it may perhaps itself 
be explained thus:—The permanent anus in Vertebrates is 
derived from the posterior end of the primitive blastopore ; on 
the formation of the neurenteric canal a ventral portion of the 
blastopore remained open as the anus, and then gradually, 
remaining functional throughout the process, travelled forwards 
on the ventral surface. The objection to this view is that such 
a phylogenetic history ought to have necessitated the existence 
of a nervous loop anterior to the actual anus, because the 
primitive blastopore was surrounded by the nerve cord. Even 
if the hypothesis just stated be correct it is certain that the 
yolk aperture in Elasmobranchs extends ventrally in front of 
the actual anus, and therefore must be independent of the 
ancestral blastopore, must be an embryonic hernia. 
