The ripe ovum is nearly spherical, and, after the removal of its capsule, is found to be unprovided with any form of protecting membrane.
My investigations on the histology of the ripe ovarian ovum have been made with the ova of the Gray Skate (_Raja batis_) only, and owing to a deficiency of material are somewhat imperfect.
The bulk of the ovum is composed of yolk spherules, imbedded in a protoplasmic matrix. Dr Alexander Schultz[70], who has studied with great care the constitution of the yolk, finds, near the centre of the ovum, a kernel of small yolk spherules, which is succeeded by a zone of spherules which gradually increase in size as they approach the surface. But, near the surface, he finds a layer in which they again diminish in size and exhibit numerous transitional forms on the way to molecular yolk granules. These Dr Schultz regards as in a retrogressive condition.
Footnote 70: _Archiv für Micro. Anat._ Vol. XI. 1875.
Another interesting feature about the yolk is the presence in it of a protoplasmic network. Dr Schultz has completely confirmed, and on some points enlarged, my previous observations on this subject[71]. Dr Schultz's confirmation is the more important, since he appears to be unacquainted with my previous investigations. In my paper (_loc. cit._), after giving a description of the network I make the following statement as to its distribution.
"A specimen of this kind is represented in Plate 14, fig. 2, _n.y_, where the meshes of the network are seen to be finer immediately around the nuclei, and coarser in the intervals. The specimen further shews, in the clearest manner, that this network is not divided into areas, each representing a cell and each containing a nucleus. I do not know to what extent this network extends into the yolk. I have never yet seen the limits of it, though it is very common to see the coarsest yolk-granules lying in its meshes. Some of these are shewn in Plate 14, fig. 2, _y.k._" [This edition, p. 65.]
Footnote 71: _Quart. Journ. Micro. Science_, Oct. 1874. [This edition, No. V.]
Dr Schultz, by employing special methods of hardening and cutting sections of the whole egg, has been able to shew that this network extends, in the form of fine radial lines, from the centre to the circumference; and he rightly states, that it exhibits no cell-like structures. I have detected this network extending throughout the whole yolk in young eggs, but have failed to see it with the distinctness which Dr Schultz attributes to it in the ripe ovum. Since it is my intention to enter fully both into the structure and meaning of this network in my account of a later stage, I say no more about it here.