In the very young embryonic ovary of the cat the columns are very small and much branched. In later embryonic stages they are frequently elongated, sometimes convoluted, and are very similar to the embryonic tubuli seminiferi. In the young stages these columns are so similar to the egg tubes (which agree more closely with Pflüger's type in the cat than in other forms I have worked at) that to any one who had not studied the development of the tissue an embryo cat's ovary at certain stages would be a very puzzling object. I have, however, met with nothing in the cat or any other form which supports Kölliker's views.
My next stage is that of a twenty-two days' embryo. Of this stage I have given two figures corresponding to those of the earlier stage (figs. 36 and 36A).
From these figures it is at once obvious that the germinal epithelium has very much increased in bulk. It has a thickness 0.1 - 0.09 mm. as compared to 0.03 mm. in the earlier stage. Its inner outline is somewhat irregular, and it is imperfectly divided into lobes, which form the commencement of structures nearly equivalent to the nests of the Elasmobranch ovary. The lobes are not separated from each other by connective tissue prolongations; the epithelium being at this stage perfectly free from any ingrowths of stroma. The cells constituting the germinal epithelium have much the same character as in the previous stage. They form an outer row of columnar cells internal to which the cells are more rounded. Amongst them a few large cells with granular nuclei, which are clearly primitive ova, may now be seen, but by far the majority of the cells are fairly uniform in size, and measure from 0.01 - 0.02 mm. in diameter, and their nuclei from 0.004 - 0.006 mm. The nuclei of the columnar outer cells measure about 0.008 mm. They are what would ordinarily be called granular, though high powers shew that they have the usual nuclear network. There is no special nucleolus. The rapid growth of the germinal epithelium is due to the division of its cells, and great masses of these may frequently be seen to be undergoing division at the same time. Of the tissue of the ovary internal to the germinal epithelium, it may be noticed that the tubuliferous tissue derived from the Malpighian bodies is no longer in contact with the germinal epithelium, but that a layer of vascular stroma is to a great extent interposed between the two. The vascular stroma of the hilus has, moreover, greatly increased in quantity.
My next stage is that of a twenty-six days' embryo, but the characters of the ovary at this stage so closely correspond with those of the succeeding one at twenty-eight days that, for the sake of brevity, I pass over this stage in silence.
Figs. 37 and 37A are representative sections of the ovary of the twenty-eighth day corresponding with those of the earlier stages.
Great changes have become apparent in the constitution of the germinal epithelium. The vascular stroma of the ovary has grown into the germinal epithelium precisely as in Elasmobranchii. It appears to me clear that the change in the relations between the stroma and epithelium is not due to a mutual growth, but entirely to the stroma, so that, as in the case of Elasmobranchii, the result of the ingrowth is that the germinal epithelium is honeycombed by vascular stroma. The vascular growths generally take the paths of the lines which separated the nests in an earlier condition, and cause these nests to become the egg tubes of Pflüger. It is obvious in figure 37 that the vascular ingrowths are so arranged as imperfectly to divide the germinal epithelium into two layers separated by a space with connective tissue and blood-vessels. The outer part is relatively thin, and formed of a superficial row of columnar cells, and one or two rows of more rounded cells; the inner layer is much thicker, and formed of large masses of rounded cells. The two layers are connected together by numerous trabeculæ, the stroma between which eventually gives rise to the connective tissue capsule, or tunica albuginea, of the adult ovary.
The germinal epithelium is now about 0.19 to 0.22 mm. in thickness. Its cells have undergone considerable changes. A fair number of them (fig. 37A, p.o.), especially in the outer layer of the epithelium, have become larger than the cells around them, from which they are distinguished, not only by their size, but by their granular nucleus and abundant protoplasm. They are in fact undoubted primitive ova with all the characters which primitive ova present in Elasmobranchii, Aves, &c. In a fairly typical primitive ovum of this stage the body measures 0.02 mm. and the nucleus 0.014 mm. In the inner part of the germinal epithelium there are very few or no cells which can be distinguished by their size as primitive ova, and the cells themselves are of a fairly uniform size, though in this respect there is perhaps a greater variation than might be gathered from fig. 37A. The cells are on the average about 0.016 mm. in diameter, and their nuclei about 0.008 to 0.001 mm., considerably larger, in fact, than in the earlier stage. The nuclei are moreover more granular, and make in this respect an approach to the character of the nuclei of primitive ova.
The germinal epithelium is still rapidly increasing by the division of its cells, and in fig. 37A there are shewn two or three nuclei in the act of dividing. I have represented fairly accurately the appearance they present when examined with a moderately high magnifying power. With reference to the stroma of the ovary, internal to the germinal epithelium, it is only necessary to refer to fig. 37 to observe that the tubuliferous tissue (t) forms a relatively smaller part of the stroma than in the previous stage, and is also further removed from the germinal epithelium.
My next stage is that of a young rabbit two days after birth, but to economise space I pass on at once to the following stage five days after birth. This stage is in many respects a critical one for the ovary, and therefore of great interest. Figure 38 represents a transverse section through the ovary (on rather a smaller scale than the previous figures) and shews the general relations of the tissues.
The germinal epithelium is very much thicker than before—about 0.38 mm. as compared with 0.22 mm. It is divided into three obvious layers: (1) an outer epithelial layer which corresponds with the pseudo-epithelial layer of the Elasmobranch ovary, average thickness 0.03 mm. (2) A middle layer of small nests, which corresponds with the middle vascular layer of the previous stage; average thickness 0.1 mm. (3) An inner layer of larger nests; average thickness 0.23 mm.