38 



BASHFOlîD DEAN. 



The germ disc is 

 not at the animal pole, 

 where it would be apt to 

 be if the lines are due to 

 cleavage. 



They may have no I have not as yet been able to prepare sec- 

 constant relation to tions of the surface of the egg. The presence 

 nuclear structures. of many nuclei in each segment, however, 



would evidently have little bearing upon this 

 point, since polyspermy undoubtedly occurs. 



There is evidence that the present position 

 of the germ disc is a secondary one, for in eggs 

 just deposited, (1) it is nearer the animal 

 pole than in later stages: ( - 2) there is a kind of 

 track, whitish in color, extending from the 

 direction of the upper pole of the egg, sugges- 

 ting therefore that the disc has shifted its 

 position, leaving a wake behind it. 



There is, however, no evidence that such dis- 

 tinct structures are ever produced by niero- 

 cytes. Moreover, there are grounds for believ- 

 ing that in the older sharks these characters 

 were less highly developed than in the more 

 recent ones. Thus it has been shown by 

 Rückert that the tendency for merocytes to 

 emigrate is less marked in a shark like Pris- 

 tiurus, than in the more specialized Torpedo : 

 that in the shark the entrance of the nuclei 

 into the yolk is less precise, and that the nuclei 

 while in the germ disc are in general more 

 superficial in position. We can justly infer 

 that in so ancient a shark as Cestracion the 

 part played by merocytes during segmentation 

 would, if anything, be less marked than in a 

 later form like Pristiurus. 



From the foregoing analysis we can, I believe, conclude reasonably 

 that the egg of Cestracion retains rudiments, at least, of its ancestral 

 cleavage. But we must also admit that absolute certainty in this con- 



Why are not these 

 blastomere-like struct- 

 ures due to merocytes, 

 like, for example, the 

 small irregular emine- 

 nces surrounding the 

 germ disc of Torpedo 

 or Pristiurus ? 



