NOTES ON ELASMOBEANOH DEVELOPMENT. 243 



Balfour's description is in the account given of the formation 

 and of the at first open condition of the hind gut. 



I quite agree with Kastschenko's remarks on the view that 

 the embryo is formed by the fusion of two separate halves. It 

 mustj however^ be admitted that the embryo is formed by a 

 bilateral growth ; that there are two growing points — one in 

 each caudal lobe, which contributes to its development. With 

 regard to the growth of the blastoderm, I agree essentially 

 with Balfour, but I differ from him as to the growth of the 

 embryo. His views are expressed in the following passage 

 ('Comp. Emb.,' 1st ed., ch. iii, p. 35; Mem. Ed,, vol. iii, 

 p. 43) : — " This rim [the embryonic rim] is a very important 

 structure, since it represents the dorsal portion of the lip of 

 the blastopore of Amphioxus. The space between it and the 

 yolk represents the commencing mesenteron, of which the 

 hypoblast on the under side of the lip is the dorsal wall. The 

 ventral wall of the mesenteron is at first formed solely of yolk, 

 held together by a protoplasmic network with numerous 

 nuclei. The cavity under the lip becomes rapidly larger, 

 owing to the continuous conversion of lower layer 

 cells into columnar hypoblast along an axial line- pass- 

 ing from the middle of the embryonic rim towards the centre 

 of the blastoderm." The italics are mine, and are used to 

 bring out the point in which my view is divergent from 

 Balfour's. He regards the embryonic rim, at its first appear- 

 ance, as marking the hind end of the future embryo, which is 

 formed by a differentiation forwards of the blastoderm, as 

 already established. I, on the other hand, regard the same 

 point as marking the extreme front end of the future animal, 

 and consider that the notched embryonic rim grows over the 

 yolk uniformly with the rest of the blastoderm edge. It 

 certainly does so extend itself, at any rate until the stage of 

 my fig. 1, and of fig. 3 also, allowing for the shoot back of the 

 caudal tongue. And it appears to me that this view — which 

 is, to a certain extent, in accordance with the view of Eoux 

 on the growth of the Amphibian embryo (' Anat. Anzeiger,' 

 vol. iii, p. 705) — must be looked upon a,s being nearer tbe 



