EARLY STAGES IN THE DEVELOPMENT OF VERTEBRATES, 217 
Selachians and Batrachians. The neural canal (nc) is by 
the stage figured x 111, completely formed in the way 
so well known in the Bird, and between the roof of the 
canal and the external epiblast a layer of mesoblast has 
already grown in. The floor of the neural canal is the same 
layer marked ep in £11, and therefore remains continuous with 
the hypoblast at 2; and when by a simultaneous process the 
roof of the neural canal and the ventral wall of the alimentary 
become formed by the folding over of one continuous layer 
(the epiblast and hypoblast continuous at the point 2), the 
two canals, viz. the neural and alimentary, are necessarily 
placed in communication at their hind ends, as is seen in the 
diagram. 
There are several important points of difference between 
E 111 and p 111. In the first place, owing to the larger size of 
the yolk-mass in E 111, the epiblast, accompanied by mesoblast, 
has not proceeded nearly so far round it as in the previous 
case. It is also worth notice that at the right as well as at the 
left end of the germinal disc the epiblast is commencing to 
erow round the yolk. ‘The yolk has, however, become sur- 
rounded to a much smaller extent on the right hand than on 
the left. Since, in the earlier stage, the epiblast became 
continuous with the hypoblast at x, it is not from sections 
obvious how this occurs. I have therefore appended a 
diagram to explain it (E’). ‘The blastoderm rests like a disc 
on the yolk and grows over it on all sides, except at the 
point where the epiblast and hypoblast are continuous 
(v). This point becomes as it were left ina bay. Next 
the two sides of the bay coalesce, the bay becomes obliterated, 
and the effect produced is exactly as if the blastoderm had 
grown round the yolk at the point # (corresponding with the 
tail of the embryo) as well as everywhere else. It thus 
comes about that the final point where the various parts of 
the blastoderm meet and completely enclose the yolk mass 
does not correspond with the anus of Rusconi of the Frog, 
but is at some little distance from the hind end of the em- 
bryo. In other words, the position of the blastopore in the 
Selachian is not the same as in the Frog. 
Another point deserving attention is the formation of the 
ventral wall of the alimentary canal. This takes place in 
two ways—partly by a folding-in at the sides and end, 
and partly from cells formed around the nuclei (z) in the 
yolk. From these a large portion of the ventral wall of the 
midgut is formed. 
The folding-in of the sheet of hypoblast to assist in the 
closing-in of the ventral wall of the alimentary canal is a 
