216 F, M. BALFOUR, 
its accompanying mesoblast has spread completely round the 
yolk material so as to form the ventral wall of the body. 
Though in some points this manner of development may 
seem to differ from that of the Frog, there is really a funda- 
mental agreement between the two, and between this mode 
of development and that of the Selachians we shall find the 
agreement to be very close. 
After segmentation we find that the egg of a selachian 
consists of two parts—one of these called the germinal disc or 
blastoderm, and the other the yolk. The former of these 
corresponds with the epiblast and the part of the lower pole 
composed of smaller segments in the last-described egg, and 
the latter to the larger segments of the lower pole. This 
latter division, owing to the quantity of yolk which it con- 
tains, has not undergone segmentation, but its homology with 
the larger segments of the previous eggs is proved (1) by its 
containing a number of nuclei (£ 1, 7), which become the nuclei 
of true cells and enter the blastoderm, and (2) by the presence 
in it of a number of lines forming a network similar to that 
of many cells. The segmentation cavity, as before, les com- 
pletely within the lower layer cells. 
The next stage, E 11,1s almost precisely similar to the 
second stage of the last egg. As there, the primitive 
involution is merely represented by a split separating the 
yolk and the germinal disc, and on the dorsal side alone 
is there a true cellular wall for this split, and at the dorsal 
mouth of the split the alimentary epithelium becomes con- 
tinuous with the epiblast. 
The segmentation cavity has become diminished, and 
round the yolk the epiblast, accompanied by a layer of meso-- 
blast, is commencing to grow. In this growth all parts of 
the blastoderm take a share except that part where the 
epiblast and hypoblast are continuous. This manner of 
growth is precisely what occurs in the Frog, though there it 
is not so easily made out; and not all the investigators who 
have studied the Frog have understood the exact meaning of 
the appearances they have seen and drawn. ‘This similarity 
of relation of the epiblast to the yolk in the two cases is a 
further confirmation of the identity of the Selachian’s yolk 
with the large yolk-spheres of the previous eggs. 
The next stage, E 111, is in many ways identical with the 
corresponding stage in the last-described egg, and in the same 
way as in that case the neural and alimentary canals are 
placed in communication with each other. 
The mode in which this occurs will be easily gathered 
from a comparison of H 11and E11, Itis the same for the 
