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E. W. M ACER IDE. 
this disc is a segmentation cavity, the Hoor of which at first 
contains some cells, but soon becomes unsegmented yolk. 
Growth and differentiation of cells leading’ to the formation 
of a dorsal lip of the blastopore occur as iisual at the edge of 
the germinal disc, and at this point there takes place an 
invagination of the lower cells giving rise to an archenteron, 
the floor of which is unsegniented yolk containing some free 
nuclei. In Chimmra, as Bashford Dean has shown (13), the 
invagination takes place, and the dorsal lip is differentiated 
not at the edge of the germ-disc, but a little way 
inside the edge, and the yolky floor of the archenteron 
contains traces of cell division (see text-fig. 7). These facts 
point conclusively, as Dean says, to the fact that the unseg- 
mented yolky portion of the egg corresponds, not to the whole 
of the large cells of the vegetable half of the frog’s egg, but 
merely to the lower portion of these. 
The lower layer cells of the germ-disc of an Elasmobranch 
correspond to the upper half of the yolky cells in the fi’Og’s 
egg. The ventral lip of the blastopore, however, is not, as 
Dean assumes, situated beneath the open blastopore. It is 
the free edge of the sheet of ectoderm which extends from 
the opposite anterior side of the germ-disc round the egg, 
and wdiich eventually encloses the yolk. The Elasmobranch 
peculiarities are two. First, this ventral lip, instead of 
extending round the sphere in a great circle, 
extends laterally round it in a small circle, and its two sides 
meet behind the embryo in a linear streak before the yolk is 
covered (see text-fig. 6). Thus the blastopore is divided into 
a dorsal and a ventral half ; the latter, called by Balfour the 
“yolk-blastopore,” corresponds to a part of the blastopore of 
the frog. The dorsal blastopore becomes closed by the medul- 
lary folds and gives rise to the neurenteric canal, but the second 
Elasmobranch peculiarity is that before this closing takes 
place, the growth of the tail region commences and 
the hinder end of the embryo presents an open groove 
leading from the anal region round to the neural tube. It is 
this phenomenon which has given rise to the idea that the 
