NOTES ON ELASMOBRANCH DEVELOPMENT. 569 
truth than Balfour’s; for if Balfour’s view is correct, the 
embryonic rim being stationary in growth backwards—all the 
differentiation being forward—ought, from the first, to be 
placed in a bay of the edge of the blastoderm. 
According to my view, then, the blastoderm grows uniformly 
over the yolk at all points of its cireumference. Indeed, its 
edge is everywhere raised into a marked ridge, which is con- 
tinuous with the embryonic rim. The difference between the 
growth at the embryonic rim and elsewhere consists in the 
fact that, as the former extends over the yolk, a trail of 
columnar epithelial cells is left separated from the yolk by a 
space, whereas elsewhere the raised edge of the blastoderm 
simply slides over the yolk, leaving, as far as one can see, little 
(possibly a few mesoderm-cells) or no trail. 
Further, it is clear, from what I have said above, that the 
notch of the embryonic rim represents the anterior end of the 
blastopore, and that on the view of embryonic growth above 
stated the blastopore does at one time or another perforate the 
whole length of the meduilary plate. Posteriorly it does 
actually form for a short time a slit through the medullary 
plate, but anteriorly it keeps closing up as the embryonic rim 
grows backwards, so that it is never present in this region as 
more than a notch. 
It will be maintained by some that this view of the growth of 
the embryo, and of the relation of the blastopore to the medul- 
lary plate, is incompatible with the objection to the concres- 
cence theory above formulated. To thisthe reply would be that 
the body of the Elasmobranch embryo is no more formed by 
the fusion of two lateral halves than is the body of the Peri- 
patus embryo, in which nearly the whole of the ventral surface 
is at one time traversed by the long blastopore. 
The phenomenon we are in both these cases dealing with is 
the closure of the blastopore; and to talk about concrescence 
and fusion of two halves is merely obscuring the real question, 
and seeking to explain a process of growth by a phrase which 
has no satisfactory meaning. 
Before leaving this part of my subject I may point out that 
VOL. XXXIII, PART IV.—NEW SER. QQ 
