CA rR 


1891.] Embryology. 1023 
The whole of the periphery of the blastoderm turns in, forming a 
layer between the periblast and the superficial ectoderm of the animal 
pole. The invaginated layer from the posterior pole gives rise to the 
mesoblast and notochord by delamination, and the remaining cells go. 
to form the midgut, the periblast taking no part whatever in the latter 
structure, but remaining encircling the yolk, and ultimately assisting to 
digest it when it begins to disappear by absorption by the liver cells. 
The blastodermic cap grows over the yolk. ‘As the blastopore 
grows smaller, the extra embryonic part of the germ-ring is pari passu 
drawn into the tail end of the embryo, and there is thus built up in 
this region a constantly increasing wall of undifferentiated cells. . 
In the bass there is no actual concrescence in the middle line; but 
the terminal notch observed in some fish, as well as the general 
considerations derived from a comparison of Teleost with Amphibia, 
warrant us in regarding the closure of the blastopore as a process 
of concres¢ence, the result of which is to establish the primitive streak. 
The entire mass of undifferentiated cells left at the tail of the embryo 
after the blastopore closes serves as a cellular material for the back- 
ward growth of the several organs. Thus, while the extra embryonic 
germ-ring, as has been insisted upon by Agassiz and Whitman, and 
Cunningham, assuredly forms part of the embryo, it does not form 
any special part; but, on the contrary, its cells eventually find their 
way into ectodermic, mesodermic, and notochordal tissues’’ of the 
tail. In other words, the author believes the two halves of the trunk 
of the embryo are not formed by concrescence of the blastopore, 
mesoderm, or any modification of it, but that all of the mesoderm 
turned in around the rim (except at the post-embryonic pole) accu- 
mulates at the upper ridge of the blastopore (in the caudal mass) and 
at its sides, and this latter differentiates into the mesoderm and its 
products in the tail. 
‘* The alimentary canal is formed from the simple endoderm lamella 
[invaginated endoderm without the parablast] by a process of folding 
along the median line. The fold is converted into a tube by the 
meeting of its lower edges. There is a solid postanal gut formed as a 
thickening of the endoderm lamella, not as a fold. At the end of the 
postanal gut is Kupffer’s vesicle, which is formed in essentially the 
same way as the permanent alimentary tube. It is scarcely necessary 
to say that Kupffer’s vesicle and the entire post-anal gut [subsequently] 
atrophy.” ‘The discovery of this vesicle was made by Kupffer in 



