HOLOBL ASTIC REMINISCENCE IN CESTRACION. 



41 



necessarily taken through an irregularly refracting mass of albumen. 

 In the first of these photographs, J, an early stage, the radial 

 arrangement of the marginal furrows is clearly marked. In K, the egg 

 has been rotated so that the flattened groove-like ends of the furrows 

 can be seen from the region of the vegetal pole. The remaining figures 

 picture the egg in the region of the animal pole, and indicate various 

 degrees of its subdivision into blastomeres. 



If we accept the foregoing observations as evidence of holoblastic 

 reminiscence, the egg of Cestracion is evidently of considerable value in 

 comparison with the cleavage characters of other ichthyopsids. We can 

 thus conclude that the great size of the eggs of other sharks was attained 

 before total cleavage became lost; and that accordingly the yolk region 

 of such eggs is directly, not indirectly, or partially, homologous with the 

 lower pole cells in other ichthyopsids. Cestracion also indicates that the 

 change in the position of the germ disc occurred before holoblastic cleav- 

 age was given up, and we have with it the suggestion that it was from 

 some new or modified physiological cause that a distinction came to arise 

 between the germ disc region and the region of the upper pole. In such 

 a functional change may have arisen an efficient cause for the disappear- 

 ance of the holoblastic type of cleavage, which up to that time had 

 continued to develop on ancient lines, pari passu with the differen- 

 tiation of the lighter and heavier deutoplasmic elements of the egg. 



Marine Laboratory of the Imperial University of Tokyo, 

 Misaki, June 8. 1901. 



Postscriptum. Misaki, July 1. 



I have recently taken several eggs (early blastula) from the oviduct 

 of Cestracion. And there can now be little doubt that the lines rep- 

 resent cleavages. In one specimen the entire region of the living germ 

 disc was successfully removed and viewed as a transparent object, and one 

 could then detect cellular outlines bridging the space between the germ 

 disc and the yolk furrows. 



B. D. 



