A fairly typical nest, selected to shew the formation of the follicle, is represented on Pl. 26, fig. 39B.
The nest contains (1) fully formed permanent ova, completely or wholly enclosed in a follicle. (2) Smaller ova, not enclosed in a follicle. (3) Smallish cells with modified nuclei of doubtful destination. (4) Small cells obviously about to form follicular epithelium.
The inspection of a single such nest is to my mind a satisfactory proof that the follicular epithelium takes its origin from the germinal epithelium and not from the stroma or tubuliferous tissue. The several categories of elements observable in such a nest deserve a careful description.
(1) The large ova in their follicles.—These ova have precisely the character of the young ova in Elasmobranchii. They are provided with a granular body invested by a delicate, though distinct membrane. Their nucleus is large and clear, but traversed by the network so fully described for Elasmobranchii. The cells of their follicular epithelium have obviously the same character as many other small cells of the nest. Two points about them deserve notice—(a) that many of them are fairly columnar. This is characteristic only of the first formed follicles. In the later formed follicles the cells are always flat and spindle-shaped in section. In this difference between the early and late formed follicles Mammals agree with Elasmobranchii. (b) The cells of the follicle are much more columnar towards the inner side than towards the outer. This point also is common to Mammals and Elasmobranchii.
Round the completed follicle a very delicate membrana propria folliculi appears to be present[412].
The larger ova, with follicular epithelium, measure about 0.04 mm., and their nucleus about 0.02 mm., the smaller ones about 0.022 mm., and their nucleus about 0.014 mm.
(2) Medium sized ova.—They are still without a trace of a follicular epithelium, and present no special peculiarities.
(3) The smaller cells with modified nuclei.—I have great doubt as to what is the eventual fate of these cells. There appear to be three possibilities.
(a) That they become cells of the follicular epithelium; (b) that they develop into ova; (c) that they are absorbed as a kind of food by the developing ova. I am inclined to think that some of these cells may have each of the above-mentioned destinations.
(4) The cells which form the follicle.—The only point to be noticed about these is that they are smaller than the indifferent cells of the germinal epithelium, from which they no doubt originate by division. This fact has already been noticed by Waldeyer.