244 ADAM SEDGWICK. 



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 circumference. 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 medullary 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 this the 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 



