Barry asserts that "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." Ne vertheless the ripe ovum which is about to be expelled, or one which has been just discharged, presents certain well-marked cha racteristics, of which the following are the most important.
The ovum is closely invested by a layer of nucleated cells. These form a portion of the granular membrane or lining of the Graafian follicle in which it is imbedded, and when the ovum is discharged from the follicle,,as described at p. 560., a portion of these gra nules is carried with it into the mouth of the Fallopian tube.
In ova which are not quite ripe, these nu cleated cells are round, but during the ces trus, in the riper ova, the cells become elon gated and fusiform, having their pointed ends attached to the zona pellucida or bounding membrane of the ovum. They present a glassy swollen aspect, by which the fully ripe for- the conjecture that they might furnish materials for the construction of the chorion has not been supported by any direct obser vations. On the contrary, numerous obser vations of Bischoff show that this process of freeing the ovum from its surrounding layer of cells, takes place very soon after its en trance within the tube, and generally in the upper third.
- ova _ acquire an appearance of being sur rounded by rays. This change occurs in most mammalia, as the dog, rabbit, sheep, rat, roe, and kangaroo. It is characteristic of the mature ova, and may be regarded as a certain sign of their ripeness.
Corresponding with this external alteration in the appearance of the ovum, are certain internal changes, of which the chief is the disappearance of the germinal vesicle. This indeed seems to be an almost constant phe nomenon throughout the mammalia, though, as to the precise mode, or even time, of dis appearance of this important constituent of the ovum, observers are by no means agreed. By Barry it was considered, after close ob servation, that the vesicle was not dissolved nor ruptured, as many now suppose, but that it became lost to observation by retiring to the centre of the ovum, where it was changed in character by an internal process of cell de velopment.
These changes, external and internal, are the precursors of impregnation, and charac terise the ovum shortly prior to and at the period of its quitting the ovary.
Arrived within the Fallopian tube, the first alteration which the ovum experiences is the stripping off of the ray-like appendage of nu cleated cells with which it quitted the ovary. This change results apparently from a burst ing and diffinence of these cells, now no longer capable of serving any useful purpose ; And now, if the coitus does not obtain, and no contact of the generative elements occurs, the ovum perishes ; observations at least relative to its further fate are wanting. But should the ovum have become fertilised, then a noticeable series of changes takes place, of which the following are the most important.
The zona pellucida, or transparent bound ing membrane of the ovum, having been freed of its external granular investment, the entire ovum presents the condition represented in figure 412. Deprived now of all encum brance, the surface of the ovum is in a condition eminently favourable for the pas sage through it of the spermatozoa, which penetrate readily that soft outer coat, and thus gain admission to the yelk.
The fact of the penetration of the outer coat of the ovum by the spermatozoa, which has been so often asserted and denied. may now, after much controversy, be considered as established. In the mammalian ovum, this passage may take place apparently through any portion of the outer coat, just as it does in the ova of amphibia, and not through a special pore or microphyle, such as exists in the ova of osseous fishes.
Following this act of penetration occurs a change which apparently affords the first distinct evidence that the power of the sper matozoa has been efficiently exerted upon the ovum. The yelk, which had previously completely filled the zona, is observed to have become contracted, so that an inter space is left between it and the zona, termed by Newport, who has carefully watched its formation in the ova of amphibia, the " re spiratory Chamber." Such a retiring of the velk. so as to leave an intcrsnace between the latter and the zona pellucida, which in terspace is filled by a transparent fluid, has been noticed in many mammalia, as the Gninea-nig. rabbit. Szc.