568 ADAM SEDGWICK. 
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 
must, 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.,’ Ist ed., ch. iii, p. 85; 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 
ceils 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. 2 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 Roux 
on the growth of the Amphibian embryo (‘ Anat. Anzeiger,’ 
vol. ii, p. 705)—must be looked upon as being nearer the 
