The very young follicles consist of but a single layer of follicular cells surrounding the ovum, which lies centrally. The follicular cells gradually increase in number until the ovum lies in the midst of several layers of cells. At this period a vacuity is formed between some of these cells, and grows into a large cell-free cavity; the ovum does not lie loosely in this space, but is connected at one side with the follicular cells, which still line the interior of the Graafian follicle by the so-called discus or cumulus proligerus. The egg or ovum has, moreover, a layer of cells immediately surrounding itself. All these facts can be gathered by an inspection of Fig. 45. It has been shown that, as in lower vertebrates, the cells immediately surrounding the ovum are connected with it directly by delicate processes which penetrate the actual membrane of the egg. [Illustration] FIG. 45.--Two stages in the development of the Graafian follicle. A, With the follicular fluid beginning to appear; B, after the space has largely increased. _caps_, Capsule; _disc_, cumulus proligerus; _memb_, membrana granulosa; _ov_, ovum; _sp_, space containing fluid. (After Hertwig.) [Illustration] FIG. 46.--Ovarian egg of _Echidna_. _b_, Basilar membrane; _fe_, follicular epithelium; _o_, oil globules; _vm_, vitelline membrane; _y_^1, _y_^2, yolk. (Partly after Caldwell.) The only ova which depart at all in structure from that above described are those of the Monotremata. The credit of this {72} discovery rests with Owen and with Professor Poulton, who pointed out in 1884,[37] that the ovum of _Ornithorhynchus_ is very large as compared with those of other Mammalia (6 mm. as against .2 mm.), that it is filled with yolk, and that it completely fills the follicle, being surrounded by two layers of follicular cells only. This latter fact was proved by Caldwell. Subsequently Gyldberg[38] and I[39] described the ovarian ovum of _Echidna_, showing it to be identical with that of _Ornithorhynchus_. Later still a more elaborate and beautifully illustrated paper was published by Caldwell[40] upon the early stages of development in the Monotremata and Marsupials, in which the ovum of the former was accurately described (see Fig. 46). In the particulars mentioned above, the ovum of the Monotremata is practically identical with that of the large-yolked ova of the Sauropsida. {73} [Illustration] FIG. 47.--_Lepus cuniculus._ The anterior end of the vagina, with the right uterus, Fallopian tube, and ovary. (Nat. size.) Part of the ventral wall of the vagina is removed, and the proximal end of the left uterus is shown in longitudinal section, _fl.t_, Fallopian tube; _fl.t_', its peritoneal aperture; _l.ut_, left uterus; _l.ut_', left os uteri; _ov_, ovary; _r.ut_, right uterus; _r.ut_', right os uteri; _s_, vaginal septum; _va_, vagina. (From Parker's _Zootomy_.)