My observations on this subject do not agree very closely with those either of Ludwig or Semper. The differences between us partly, though not entirely, depend upon the fundamentally different views we hold about the constitution of the ovary and the nature of the epithelium covering it (vide pp. [555] and [556]).
In very young ovaries (Pl. 24, fig. 8) nests of ova (in my sense of the term) are very numerous, but though usually superficial in position are also found in the deeper layers of the ovary. They are especially concentrated in their old position, close to the dorsal edge of the organ. In some instances they do not present quite the same appearance as in the embryo, owing to the outlines of the ova composing them being distinct, and to the presence between the ova of numerous interstitial cells derived from the germinal epithelium, and destined to become follicular epithelium. These latter cells at first form a much flatter follicular epithelium than in the embryonic periods, so that the smaller adult ova have a much less columnar investment than ova of the same size in the embryo. A few primitive ova may still be found in a very superficial position, but occasionally also in the deeper layers. I am inclined to agree with Semper that some of these are freshly formed from the cells of the germinal epithelium.
In the young female with ova of about 0.5 mm. nests of ova are still fairly numerous. The nests are characteristic, and present the various remarkable peculiarities already described in the embryo. In many instances they form polynuclear masses, not divided into separate cells, generally, however, the individual ova are distinct. The ova in these nests are on the average rather smaller than during the embryonic periods. The nests are frequently quite superficial and at times continuous with the pseudo-epithelium, and individual ova also occasionally occupy a position in the superficial epithelium. Some of the appearances presented by separate ova are not unlike the figures of Ludwig, but a growth such as he describes has, according to my observations, no existence. The columns which he believes to have grown into the stroma are merely trabeculæ connecting the deeper and more superficial parts of the germinal epithelium; and his whole view about the formation of the follicular epithelium round separate ova certainly does not apply, except in rare cases, to Scyllium. It is, indeed, very easy to see that most freshly formed ova are derived from nests, as in the embryo; and the formation of a follicular epithelium round these ova takes place as they become separated from the nests. A few solitary ova, which have never formed part of a nest, seem to be formed in this stage as in the embryo; but they do not grow into the stroma surrounded by the cells of the pseudo-epithelium, and only as they reach a not inconsiderable size is a definite follicular epithelium formed around them. The follicular epithelium, though not always formed from the pseudo-epithelium, is of course always composed of cells derived from the germinal epithelium.
In all the ova formed at this stage the nucleus would seem to pass through the same metamorphosis as in the embryo.
In the later stages, and even in the full-grown female of Scyllium, fresh ova seemed to be formed and nests also to be present. In Raja I have not found freshly formed ova or nests in the adult, and have had no opportunity of studying the young forms.
Summary of observations on the development of the ovary in Scyllium and Raja.
(1) The ovary in the embryo is a ridge, triangular in section, attached along the base. It is formed of a core of stroma and a covering of epithelium. A special thickening of the epithelium on the outer side forms the true germinal epithelium, to which the ova are confined (Pl. 24, fig. 1). In the development of the ovary the stroma becomes differentiated into an external vascular layer, especially developed in the neighbourhood of the germinal epithelium, and an internal lymphatic portion, which forms the main mass of the ovarian ridge (Pl. 24, figs. 2, 3, and 6).
(2) At first the thickened germinal epithelium is sharply separated by a membrane from the subjacent stroma (Pl. 24, figs. 1, 2, and 3), but at about the time when the follicular epithelium commences to be formed round the ova, numerous strands of stroma grow into the epithelium, and form a regular network of vascular channels throughout it, and partially isolate individual ova (Pl. 24, figs. 7 and 8). At the same time the surface of the epithelium turned towards the stroma becomes irregular (Pl. 24, fig. 9), owing to the development of individual ova. In still later stages the stroma ingrowths form a more or less definite tunic close to the surface of the ovary. External to this tunic is the superficial layer of the germinal epithelium, which forms what has been spoken of as the pseudo-epithelium. In many instances the protoplasm of its cells is produced into peculiar fibrous tails which pass into the tunic below.
(3) Primitive ova.—Certain cells in the epithelium lining the dorsal angle of the body-cavity become distinguished as primitive ova by their abundant protoplasm and granular nuclei, at a very early period in development, even before the formation of the genital ridges. Subsequently on the formation of the genital ridges these ova become confined to the thickened germinal epithelium on the outer aspect of the ridges (Pl. 24, fig. 1).
(4) Conversion of primitive ova into permanent ova.—Primitive ova may in Scyllium become transformed into permanent ova in two ways—the difference between the two ways being, however, of secondary importance.