Formation of the permanent ova and the follicular epithelium.—In my monograph on the development of Elasmobranch Fishes an account was given of the earliest stages in the development of the primitive ova, and I now take up their development from the point at which it was left off in that work. From their first formation till the stage spoken of in my monograph as P, their size remains fairly constant. The larger examples have a diameter of about 0.035 mm., and the medium-sized examples of about 0.03 mm. The larger nuclei have a diameter of about 0.16 mm., but their variations in size are considerable. If the above figures be compared with those on page 350 of my monograph on Elasmobranch Fishes, it will be seen that the size of the primitive ova during these stages is not greater than it was at the period of their very first appearance.

The ova (Pl. 24, fig. 1) are usually aggregated in masses, which might have resulted from division of a single ovum. The outlines of the individual ova are always distinct. Their protoplasm is clear, and their nuclei, which are somewhat passive towards staining reagents, are granular, with one to three nucleoli. I have noticed, up to stage P, the occasional presence of highly refractive spherules in the protoplasm of the primitive ova already described in my monograph (pp. [353], [354], Pl. 12, fig. 15). They seem to occur up to a later period than I at first imagined. Their want of constancy probably indicates that they have no special importance. Professor Semper has described similar appearances in the male primitive ova of a later period.

As to the distribution of the primitive ova in the germinal epithelium, Professor Semper's statement that the larger primitive ova are found in masses in the centre, and that the smaller ova are more peripherally situated is on the whole true, though I do not find this distribution sufficiently constant to lay so much stress on it as he does.

The passive condition of the primitive ova becomes suddenly broken during stage Q, and is succeeded by a period of remarkable changes. It has only been by the expenditure of much care and trouble that I have been able to elucidate to my own satisfaction what takes place, and there are still points which I do not understand.

Very shortly after stage Q, in addition to primitive ova with a perfectly normal nucleus, others may be seen in which the nucleus is apparently replaced by a deeply stained irregular body, smaller than the ordinary nuclei (Pl. 24, fig. 11, d.n.). This body, by the use of high objectives, is seen to be composed of a number of deeply stained granules, and around it may be noticed a clear space, bounded by a very delicate membrane. The granular body usually lies close to one side of this membrane, and occasionally sends a few fine processes to the opposite side.

The whole body, i.e. all within the delicate membrane is, according to my view, a modified nucleus; as appears to me very clearly to be shewn by the fact that it occupies the normal position of a nucleus within a cell body. Semper, on the other hand, regards the contained granular body as the nucleus, which he compares with the spindles of Bütschli, Auerbach, &c.[374]. This interpretation appears to me, however, to be negatived by the position of these bodies. The manner in which Semper may, perhaps, have been led to his views will be obvious when the later changes of the primitive ova are described. The formation of these nuclei would seem to be due to a segregation of the constituents of the original nuclei; the solid parts becoming separated from the more fluid. As a rule, the modified nuclei are slightly larger than the original ones. In stage Q the following two tables shew the dimensions of the parts of three unmodified and of three modified nuclei taken at random.

Primitive ova with unmodified nuclei—

Nuclei.
0.014 mm.
0.012 mm.
0.01 mm.

Primitive ova with modified nuclei—

Nuclei.Granular
Bodies in nuclei.
0.018 mm.0.006 mm.
0.018 mm.0.006 mm.
0.012 mm.0.009 mm.