My own observations in conjunction with the specially interesting observations of Dr Schultz[156] 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 throughout the whole yolk, is present both before and after impregnation. Footnote 156: _Archiv f. Mikr. Anat._ Vol. XXI. 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 Elasmobranchii 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 Elasmobranchii, 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 protoplasm than they do in the Amphibian ovum. As I have suggested elsewhere[157], 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 approached, segmentation becomes slower, and the resulting segments larger and larger. Footnote 157: "Comparison," &c., _Quart. Journ. Micr. Science_, July, 1875. [This Edition, No. VI.] 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 interest. An answer to it has already been attempted from a general point of view in my paper[158] entitled 'The Comparison of the early stages of development in Vertebrates'; but the subject may be conveniently treated here in a special manner for Elasmobranch embryos. Footnote 158: _Loc. cit._