HOLOBLASTIC REMINISCENCE IN CESTRACION. 



37 



Keasons for regarding the lines on the egg of Cestracion as remin- 

 iscent of holoblastic cleavage. 



Unfavorable evidence. 



Favorable evidence. 



Such lines have never 

 been known to occur in 

 the egg of any other 

 shark. 



They may be surface 

 wrinkles merely. 



They have not been 

 traced back to the ear- 

 liest cleavage lines. 



Their occurrence, therefore, is all the more 

 significant in Cestracion, on account of the 

 close relationship of this form with palaeo- 

 zoic genera. Cf. also the breaking up of the 

 egg of Chimaera into lobes or blastomeres. 



In this event they should be superficial, 

 more or less inconstant, and subject to change 

 by artificial means. They are, however, not 

 only present in all early stages examined, but 

 they suggest distinctly the outlines of blasto- 

 meres. They show no change for as long a 

 time as I have been able to keep alive the 

 opened egg — a matter of 30 hours. They 

 can be distinguished after the egg has been 

 broken, and at such a time it is seen that they 

 are not shallow surface markings, but that 

 they pass deeply into the yolk ; they are 

 deepest (1. 5 mm.) at the upper pole, and 

 become shallower as they pass downward. 



The most serious objection, for as yet I have 

 failed to secure the earliest stages of segmen- 

 tation. The egg of Cestracion does not ex- 

 hibit these lines shortly before it sepai'ates 

 from the ovary. They are present when 

 the egg is deposited (late segmentation) and 

 they remain conspicuous for as long a time 

 (over a month) as the yolk is uninclosed. 

 In the earlier stages, finally, they are generally 

 less numerous than in later ones. 



