270 DEVELOPMENT OF ELASMOBRANCH FISHES. 



question arises whether in the case of meroblastic ova the cell 

 is not constituted of two parts completely separated from one 

 another. 



Is the meroblastic ovum, before or after impregnation, com- 

 posed of a germinal disc in which all the protoplasm of the cell 

 is aggregated, and of a food-yolk in which no protoplasm is 

 present ? or is the protoplasm present throughout, being simply 

 more concentrated at the germinal pole than elsewhere ? If the 

 former alternative is accepted, we must suppose that the mass of 

 food-yolk is a something added which is not present in holoblas- 

 tic ova. If the latter alternative is accepted, it may then be 

 maintained that holoblastic and meroblastic ova are constituted 

 in the same way and differ only in the proportions of their con- 

 stituents. 



My own observations in conjunction with the specially inte- 

 resting observations of Dr Schultz 1 justify the view which regards 

 the protoplasm as present throughout the whole ovum, and not 

 confined to the germinal disc. Our observations shew that a 

 fine protoplasmic network, with ramifications extending through- 

 out the whole yolk, is present both before and after impregna- 

 tion. 



The presence of this network is, in itself, only sufficient to 

 prove that the yolk may be equivalent to part of a holoblastic 

 ovum ; to demonstrate that it is so requires something more, and 

 this link in the chain of evidence is supplied by the nuclei of the 

 yolk, which have been so often referred to. 



These nuclei arise independently in the yolk, and become 

 the nuclei of cells which enter the germ and the bodies of which 

 are derived from the protoplasm of the yolk. Not only so, but 

 the cells formed around these nuclei play the same part in the 

 development of Elasmobranchs as do the largest so-called yolk 

 cells in the development of Amphibians. Like the homologous 

 cells in Amphibians, they mainly serve to form the ventral wall 

 of the alimentary canal and the blood-corpuscles. The identity 

 in the fate of the so-called yolk cells of Amphibians with the cells 

 derived from the yolk in Elasmobranchs, must be considered 

 as a proof of the homology of the yolk cells in the first case 



1 Archivf. Mikr. Anat. Vol. xxi. 



