1026 } The American Naturalist. [November, 
made out.’ This argument itself is vitiated in the assumption of a 
fixed point for the oil globule, as the author gives little reason for 
believing that the latter may not change position with the growth of 
the embryo. Moreover, and this is much more important, the author’s 
conception as to the position of the frog’s gastrula in relation to the 
poles of the embryo may not be correct, and must seriously alter his con- 
ception of the position of the fish embryo to the egg if he insists on a 
strict comparison of the two forms. (See below.) 
The author’s conception of gastrulation in the fish may be gathered 
from the following quotation: ‘‘ Accepting Ziegler’s homologies, it 
will be seen that the whole course of the fish development becomes 
easy to understand. Starting with the blastula, and disregarding for 
the present the non-embryonic part of the germ-ring, the primitive 
hypoblast corresponds to the primitive hypoblast which invaginates 
around the dorsal lip of the blastopore in the frog gastrula. The chief 
point of difference is the lack of continuity in the fish embryo between 
the inner edge of the invaginated layer, and the yolk is easily explained 
as an adaptation to the method of forming the alimentary canal from 
the invaginated layer exclusively. The archenteron lies between the 
primitive hypoblast and the periblast. In consequence of the absence 
of continuity between the yolk and the invaginated layer, the archen- 
teron at its edge is not separated from the segmentation cavity Zhe 
growth of the anterior pole of the blastoderm around the yolk represents 
the growth of smail cells around the yolk-cells in amphibian gastrulation. 
The closure of the blastopore takes place in the same way as in the 
Amphibia ; there is formed a short primitive streak behind the position 
of the neurenteric canal (Kupffer’s vesicle in Teleosts) ; at the posterior 
endof the primitive streak the final closure takes plac Th 
asymmetry which Balfour showed to. be a Ser ka miia 
vertebrate gastrulation is present in the highest degree in the Teleost 
gastrula. At the posterior pole of the blastoderm (dorsal lip of blasto- 
pore) there is an extensive invagination which gives rise to the roof of the 
archenteron, The cause of the asymmetry must be looked for in the 
peculiarly localized distribution of the yolk in the egg.” 
_ The italics above are my own, and emphasize the fact that the 
author does not realize that in the gastrulation of the Amphibia, to which 
hę so constantly refers, the anterior pole does not grow over the yolk in 
an epibolic fashion, but that the anterior (black) extension of the 
blastoderm over the yolk takes place by a delamination of ectoderm 
cells from the large yolk-containing cells (see Am. Nat., Embryology, 
August, 1891). This latter view has been recognized by Kupffer and 





