RELATIONS OF YOLK TO GASTRULA IN TELEOSTEANS. 25 
the posterior end of the blastopore is included in the floor of 
the medullary canal, remaining open for some time as the neu- 
rentericcanal. There is no doubt that the embryo is formed by 
the concrescence of the lips of the blastopore in the Amphibian 
as in the Teleostean, but the invaginated ring is not so con- 
spicuous in the former, because the invagination of dorsal 
hypoblast takes place gradually pari passu with the closing of 
the blastopore, while in the Teleostean the invagination is 
completed before the closing of the blastopore begins. 
In all this no mention has been made of the formation of the 
neurochord, because this is a process which is phylogenetically 
and theoretically altogether independent of the closing of the 
blastopore. In the Teleostean the thickening of the epiblast 
which forms the neurochord coincides with the concrescence of 
the lips of the blastopore, and the thickened column extends 
down to the edge of the blastoderm, forming the greater part 
of the thickness of the embryo in the stages shown in figs. 5 
to 9. This thickening may begin in the germinal ring before 
its concrescence, and Ryder states that in Elecate even meta- 
meric segmentation of the mesoblast occurs in the germinal 
ring before it forms part of the embryo; but such facts as these 
are easily understood on the principal of abbreviation of de- 
velopment, and do not affect the truth that the concrescence of 
the edge of the blastoderm is simply the closing of the blasto- 
pore. The neural cord was originally simply a thickening of 
epiblast round the elongated blastopore or gastrula mouth, and 
the formation of the medullary canal began in the course of 
evolution after the original blastopore had closed up. It is 
probable that in Teleostei the cells which afterwards line the 
neural canal attain their internal position during the concres- 
cence, and this supposition is of interest as explaining why no 
actual inflection from the surface can be made out in the de- 
velopment of the solid neurochord. The formation of the 
notochord also, in all cases, closely follows the closing of the 
blastopore. 
I would point out here that the primitive streak described in 
Amphibian embryo, especially in the Triton by Miss Johnson 
