DEVELOPMENT OF ELASMOBRANCH FISHES. 57 



My own observatioDS in conjunction with the specially inter- 

 esting observations of Dr Schultz^ justify the view which regards 

 the protoplasm as present throughout the whole ovum, and not 

 confined to the germinal disc. Our observations show 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 

 with the yolk in the second ; the difference between the yolk in 

 the two cases arising from the fact that in the Elasmobranch 

 ovum the yolk spherules bear a larger proportion to the proto- 

 plasm than they do in the Amphibian ovum. As I have 

 suggested elsewhere'', the segmentation or non-segmentation of 

 a particular part of the ovum depends solely upon the proportion 

 borne by the protoplasm to the yolk particles; so that, when 

 the latter exceed the former in a certain fixed proportion, 

 segmentation is no longer possible; and, as this limit is ap- 

 proached, segmentation becomes slower, and the resulting 

 segments larger and larger. 



The question how far the facts in the developmental history 

 of the various vertebrate blastoderms accord with the view of 

 the nature of the yolk just propounded, is one of considerable 



1 Arcliiv f. Mikr. Anat. Vol. xxi. 



- Comparison, &c., Quart. Journ. Micr. Science, July, 1875, 



