1886. | Embryology. 471 
disk may be hardened zm sitų without distortion, and afterwards 
separated from and carefully lifted off of the underlying vitellus, 
together with a thin hardened flake of the latter to support it. 
Such was the treatment to which the blastodisk here figured and 
described was subjected. The surface view, Fig. 1, was drawn 
with the camera lucida after hardening, and the section shown in 
Fig. 2 was drawn from one taken at about the position of the line 
ain Fig. 1. Cleavage had already advanced so far as to subdi- 
vide the area of the blastodisk into fifteen sharply defined cells, 
So that it may be assumed that this blastodisk has nearly com- 
pleted its sixteen-celled stage of development or that the fourth 
cleavage is about completed. 
_A comparison of the first four cleavage planes of this blasto- 
disk shows that they are formed in very nearly the same order 
and relation to each other in Elasmobranchs as in Teleosts. For 
example, the first plane 1, in Fig. 1, has cut through the originally 
circular blastodisk and caused it to become elongated at right 
angles to the direction of the first segmentation furrow exactly 
as in the eggs of teleostean fishes. The second furrow, II, cuts 
the first at right angles so as to further subdivide the first two 
cells into four. The next cleavage is caused by two nearly par- 
allel furrows, III, 111, which appear simultaneously, and further 
subdivide the cells of the blas- 
todisk into eight. The fourth 
cleavage i 
-celled stage of teleostean d 
it is also evident 
