Page:Proceedings of the Royal Society of London Vol 4.djvu/161

Rh supposed to be possessed of the early stages in the developement of that ovum, consists chiefly of inferences from observations made on the ovum of the bird.

But there exists a period in the history of the ovum of the mam- mal, regarding which we have hitherto scarcely any direct or posi- tive knowledge. It appeared, therefore, highly desirable to obtain a series of observations in continuous succession on the earliest stages of developement. In conducting this investigation, the author pur- posely confined his attention to a single species, namely, the rabbit, of which he examined more than a hundred individual animals. Besides ova met with in the ovary, apparently impregnated, and destined to be discharged from that organ, he has seen upwards of three hundred ova in the Fallopian tube and uterus ; very few of the latter exceeding half a line in their diameter. The results of these investigations have compelled the author to express his dissent from some of the leading doctrines of embryology, which at present prevail, as respects not only the class Mammalia, but the animal kingdom at large. The following are the principal facts which the author has observed in the developement of the mammiferous ovum.

The difference between the mature and immature ovum consists in the condition of the yelk ; the yelk of the mature ovum contain- ing no oil-like globules. Both maceration and incipient absorption produce changes in the unimpregnated ovum, which in some respects resemble those referable to impregnation. During the rut, the num- ber of Graafian vesicles appearing to become prepared for dischar- ging their ova, exceeds the number of those which actually discharge them. Ova of the rabbit which are destined to be developed, are in most instances discharged from the ovary in the course of nine or ten hours post coitum ; and they are all discharged about the same time.

There is no condition of the ovum uniform in all respects which can be pointed out as the particular state in which it is discharged from the ovary ; but its condition is in several respects very different from that of the mature ovum ante coitum. Among the changes occurring in the ovum before it leaves the ovary, are the follow- ing : viz. the germinal spot, previously on the inner surface, passes to the centre of the germinal vesicle ; the germinal vesicle, previously at the surface, returns to the centre of the yelk ; and the membrane investing the yelk, previously extremely thin, suddenly thickens. Such changes render it highly probable that the ovary is the usual seat of impregnation. The author considers this view as being not incompatible with the doctrine that contact between the seminal fluid and the ovum is essential to impregnation, since he has found, in the course of his researches, that spermatozoa penetrate as far as to the surface of the ovary. The retinacula and tunica granulosa are the parts acted upon by the vis a tergo, which expels the ovum from the ovary. These parts are discharged with the ovum, render its escape gradual, probably facilitate its passage into the Fallopian tube, and appear to be the bearers of fluid for the immediate imbibition of the ovum. After the discharge of the ovum from the ovary, the ovisac is obtainable free from the vascular covering, which, together