472 General Notes. _ [May, 
velupment of the eggs of the Elasmobranchs. The subdivision 
of the blastodisk into cells in both types of Ichthyes is essentially 
a symmetrical one, determined by the first cleavage plane. 
Whether or not the first cleavage plane of the ovum of elasmo- 
branchs coincides with the median plane of the future embryo, 
as supposed by Whitman, Roux, Pflüger and E. Van Beneden, it 
is impossible to decide at present, but it would seem not at al 
_ improbable that such might be the case. 
series of sections of this blastodisk of Raia, prepared by the 
aid of a Cambridge rocking microtome, which was presented to 
the U. S. Fish Commission by Professor Adam Sedgwick, have 
enabled me to reach some interesting conclusions in reference to 
the structure of the blastoderm of the Elasmobranchii during its 
one-layered condition. At this stage the four median or central 
cells are not completely sundered from the underlying periblast, 
P, Fig. 2, since the cleavage furrows are found to terminate ab- 
-ruptly. before they have quite cut through the finely granular 
plasma of the blastodisk proper, as shown in Fig. 2. In this 
respect the cleavage of the blastodisk of Elasmobranchs differs 
very decidedly from that of teleosts as described by Agassiz and 
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