The stroma ingrowths form the most important feature of the stage. In most instances they are very thin and delicate, and might easily be overlooked, especially as many of the cells in them are hardly to be distinguished, taken separately, from those of the germinal epithelium. These features render the investigation of the exact relation of the stroma and epithelium a matter of some difficulty. I have, however, been greatly assisted by the investigation of the ovary of a young example of Scyllium stellare, 16½ centimètres in length, a section of which is represented in Pl. 25, fig. 26. In this ovary, although no other abnormalities were observable, the stroma ingrowths were exceptionally wide; indeed, quite without a parallel in my series of ovaries in this respect. The stroma most clearly divides up the epithelium of the ovary into separate masses, or more probably anastomosing columns, the equivalents of the egg-tubes of Pflüger. These columns are formed of normal cells of the germinal epithelium, which enclose ovarian nests and ova in all stages of development. A comparison of the section I have represented, with those from previous stages, appears to me to demonstrate that the relation of the epithelium and stroma has been caused by an ingrowth or penetration of the stroma into the epithelium, and not by a mutual intergrowth of the two tissues. Although the ovary, of which fig. 26 represents a section was from Scy. stellare, and the previous ovaries have been from Scy. canicula, yet the thickness of the epithelium may still be appealed to in confirmation of this view. In the previous stage the thickness was about 0.096 mm., in the present one it is about 0.16 mm., a difference of thickness which can be easily accounted for by the growth of the individual ova and the additional tracts of stroma. A pseudo-epithelium is more or less clearly formed, but it is continuous with the columns of epithelium. In the stroma many isolated cells are present, which appear to me, from a careful comparison of a series of sections, to belong to the germinal epithelium.
The thickness of the follicular epithelium on the inner side of the larger ova deserves to be noted. Its meaning is discussed on p. [567].
Quite a different interpretation to that which I have given has been put by Ludwig and Semper upon the parts of the ovary at this stage. My pseudo-epithelium is regarded by them as forming, together with the follicular epithelium of the ova, the sole remnant of the original germinal epithelium; and the masses of cells below the pseudo-epithelium, which I have attempted to shew are derived from the original germinal epithelium, are regarded as parts of the ingrowths of the adjacent stroma.
Ludwig has assumed this interpretation without having had an opportunity of working out the development of the parts, but Semper attempts to bring forward embryological proofs in support of this position.
If the series of ovaries which I have represented be examined, it will not, I think, be denied that the general appearances are very much in favour of my view. The thickened patch of ovarian epithelium can apparently be traced through the whole series of sections, and no indications of its sudden reduction to the thin pseudo-epithelium are apparent. The most careful examination that I have been able to make brings to light nothing tending to shew that the general appearances are delusive. The important difference between us refers to our views of the nature of the tissue subjacent to the pseudo-epithelium. If my results be accepted, it is clear that the whole ovarian region is an epithelium interpenetrated by connective tissue ingrowths, so that the region below the pseudo-epithelium is a kind of honeycomb or trabecular net-work of germinal epithelium, developing ova of all stages and sizes, and composed of cells capable of forming follicular epithelium for developing ova. Ludwig figures what he regards as the formation of the follicular epithelium round primitive ova during their passage into the stroma. It is quite clear to me, that his figures of the later stages, 33 and 34, represent fully formed permanent ova surrounded by a follicular epithelium, and that their situation in contact with the pseudo-epithelium is, so to speak, an accident, and it is quite possible that his figures 31 and 32 also represent fully formed ova; but I have little hesitation in asserting that he has not understood the mode of formation of the follicular epithelium, and that, though his statement that it is derived from the germinal epithelium is quite correct, his account of the process is completely misleading. The same criticism does not exactly apply to Semper's statements. Semper has really observed the formation of the follicular epithelium round young ova; but, nevertheless, he appears to me to give an entirely wrong account of the relation of the stroma to the germinal epithelium. The extent of the difference between Semper's and my view may perhaps best be shewn by a quotation from Semper, loc. cit., 465:—“In females the nests of primitive ova sink in groups into the stroma. In these groups one cell enlarges till it becomes the ovum, the neighbouring cells increase and arrange themselves around the ova as follicle cells.”
Although the histological changes which take place in the succeeding stages are not inconsiderable, they do not involve any fundamental change in the constitution of the ovarian region, and may be described with greater brevity than has been so far possible.
In a half-grown female, with an ovarian region of 3mm. in breadth, and 0.8mm. in thickness, the stroma of the ovarian region has assumed a far more formed aspect than before. It consists (Pl. 24, fig. 10) of a basis in most parts fibrous, but in some nearly homogeneous, with a fair number of scattered cells. Immediately below the pseudo-epithelium, there is an imperfectly developed fibrous layer, forming a kind of tunic, in which are imbedded the relatively reduced epithelial trabeculæ of the previous stages. They appear in sections as columns, either continuous with or independent of the pseudo-epithelium, formed of normal cells of the germinal epithelium, nests of ova, and permanent ova in various stages of development. Below this there comes a layer of larger ova which are very closely packed. A not inconsiderable number of the larger ova have, however, a superficial situation, and lie in immediate contact with the pseudo-epithelium. Some of the younger ova, enclosed amongst epithelial cells continuous with the pseudo-epithelium, are very similar to those figured by Ludwig. It is scarcely necessary to insist that this fact does not afford any argument in favour of his interpretations. The ovarian region is honeycombed by large vascular channels with distinct walls, and other channels which are perhaps lymphatic.
The surface of the ovarian region is somewhat irregular and especially marked by deep oblique transverse furrows. It is covered by a distinct, though still irregular pseudo-epithelium, which is fairly columnar in the furrows but flattened along the ridges. The cells of the pseudo-epithelium have one peculiarity very unlike that of ordinary epithelial cells. Their inner extremities (vide fig. 10) are prolonged into fibrous processes which enter the subjacent tissue, and bending nearly parallel to the surface of the ovary, assist in forming the tunic spoken of above. This peculiarity of the pseudo-epithelial cells seems to indicate that they do not essentially differ from cells which have the character of undoubted connective tissue cells, and renders it possible that the greater part of the tunic, which has apparently the structure of ordinary connective tissue, is in reality derived from the original germinal epithelium, a view which tallies with the fact that in some instances the cells of the tunic appear as if about to assist in forming the follicular epithelium of some of the developing ova. In Raja, the similarity of the pseudo-epithelium to the subjacent tissue is very much more marked than in Scyllium. The pseudo-epithelium appears merely as the superficial layer of the ovarian tunic somewhat modified by its position on the surface. It is formed of columnar cells with vertically arranged fibres which pass into the subjacent layers, and chiefly differ from the ordinary fibres in that they still form parts of the cell-protoplasm enclosing the nucleus. In Pl. 25, fig. 34, an attempt is made to represent the relations of the pseudo-epithelium to the subjacent tissue in Raja. Ludwig's figures of the pseudo-epithelium of the ovary, in the regular form of its constituent cells, and its sharp separation by a basement membrane from the tissue below, are quite unlike anything which I have met with in my sections either of Raja or Scyllium.
Close to the dorsal border of the ovary the epithelial cells of the non-ovarian region have very conspicuous tails, extending into a more or less homogeneous substance below, which constitutes a peculiar form of tunic for this part of the ovarian ridge.
In the full-grown female the stroma of the ovarian region is denser and has a more fibrous aspect than in the younger animal. Below the pseudo-epithelium it is arranged in two or three more or less definite layers, in which the fibres run at right angles. It forms a definite ovarian tunic. The pseudo-epithelium is much more distinct, and the tails of its cells, so conspicuous in previous stages, can no longer be made out.