With regard to the time at which the opening of the ovarian vesicles takes place, there are considerable varieties in the same and in diffe rent animals. In the sheep, the vesicle has been found burst so early as at two hours after coition. In the dog, Haller found the vesicles burst before the sixth day ; in one instance the day after coition ; but Prevost and Dumas, not until the seventh or eighth. In the rabbit, Cruikshank observed vesicles burst two hours after coition, while Haighton considers forty eight hours as the usual time at which the rup ture happens in this animal. AI. Coste has observed it most frequently between the second and third day in the rabbit.
After the bursting of the Graafian vesicles, there occur in them and iu the neighbouring Fut of the ovary some important changes of structure, which claim our attention in this place as intimately connected with that part of the process of conception which is now under consideration.
If the Graafian vesicle which is enlarged from venereal excitement and is ready to burst, be examined with care, it will be seen that at the most prominent part of its coats the blood vessels converge towards the point at which the rupture afterwards takes place, and this point is itself comparatively destitute of blood vessels.• At the time of the formation of the opening into the vesicle, from the division of some of the bloodvessels, a small quantity of blood is generally mixed with the fluid contents of the vesicle; and after the vesicle has been emptied of these fluid contents, their place is generally supplied by a greater or less quantity of coagu lated blood, probably poured out by the same ruptured vessels.
The membranes of the vesicle at this time have become thicker than before: the inner one in particular appears more vascular and uneven, perhaps in part from its being puckered up on the vesicle becoming flaccid and com paratively empty. The wrinkled appearance on the inner surface of the vesicle increases, and there grows gradually out from it a new substance which comes to occupy the whole cavity of the vesicle; and in many instances, as this new substance is formed in greater quantity than can be contained within the limits of the vesicle, it protrudes some way out at the opening of the vesicle, forming a dark red prominence like a nipple, which rises above the neighbouring surface of the ovary. This substance, at the time of its first formation, is of a pink or reddish colour, but as it becomes gradually less filled with blood it acquires a yellowish hue, which is more or less apparent in different animals. In the human species it is of a bright yellow colour, whence the name of corpus lute= applied to this new produc tion of the ovarian vesicles.
The substance of the corpus luteum has a lobular structure; the lobules radiating in a somewhat irregular manner from the centre to the circumference. The central part of the corpus luteum frequently remains hollow for some time after its production, opening ex teriorly by a narrow passage from the place where the rupture of the vesicle originally look place; at other times this passage is closed more early, and there remains nothing but an indication of its place in a depression in the centre of the most projecting part of the corpus luteum. The lobules of the corpus luteum, examined with the microscope, exhibit merely a granular structure, and are not formed of mini, as some have described them, so that there is no reason to consider these bodies as of a glandular nature.
The size which corpora lutea attain when fully developed varies much in the same and in different animals. In the human female they become as large as a common hazel-nut; in the cow they are sometimes as large as a ches nut; and in the sow or ewe they are somewhat larger than full-grown peas.
The corpus luteum may, by dissection, be easily separated from the surrounding parts and turned out of the ovary ; and when this is done, the external membrane of the original vesicle remains lining the cavity left in the ovary. From this it would appear that the corpus luteum is most intimately connected with the inner membrane of the vesicle; and, in fact, Baer observed that, before the rupture of the vesicle in the dog, the inner membrane had become thickened, rugous, and of a villous structure, as if the corpus luteum grew from that internal membrane itself. This observa tion also makes it probable that the growth of the corpus luteum may contribute to cause the rupture of the vesicle.
The corpus luteum at first increases gradu ally in size, remains for a time stationary, and then decreases till it either wholly disappears or leaves only a small mark or cicatrix to indi cate its place. The time at which it attains its full size seems to vary considerably. In the sheep two or three days are sufficient for the formation of the corpus luteum, and its cavity becomes obliterated within a fortnight after copulation. Ilaller found corpora lutea in the dog on the sixth day ; Cruikshank observed the corpora lutea to go on progres sively increasing till the ninth day in the rabbit ; and it is probable that in the human species the corpus luteum is not fully developed till after the second month of pregnancy.