FERTILIZATION OF OVUM.
On arriving at the ovum the spermatozoa bury themselves in the zone pellucida, and in Fig. 91 a number of them are shown in this position. One of them goes further; its head penetrates into the ovum itself, and separates from the tail, which remains outside and ultimately disap pears. The head, once inside the ovum, increases in size, assumes a radiate appearance, and is known as the male pronucleus. The male and female pronuclei—the latter, it will be remembered, being the por tion of the original germinal vesicle which has remained in the ovum— now gradually approach one another, and ultimately fuse to form the definite nucleus of the fertilized egg.' After the formation of this definite nucleus, the ovum regains the power of multiplication by fission, which it had previously lost by the formation of the polar bodies; in other words, the act of impregnation consists in the replacement by the head of the spermatozoon of that portion of the original germinal vesicle, or nucleus of the ovum, which had at an earlier stage been extruded bodily from the ovum in the form of the polar bodies.
It has been stated that primitive ova occur in the male as well as the female embryo. In the female they become converted, as already ex plained, into the permanent ova; and in the male they give rise by a series of changes to the spermatozoa; so that from these primitive ova the essential reproductive elements of both sexes are derived; the main difference between them being, that while in the female each primitive. ovum becomes a single permanent ovum, in the male each primitive ovum gives rise to a considerable number of spermatozoa.
We thus see that the male and female elements, the spermatozoa and ova, are fundamentally very similar to one another; and if we reflect far ther that the head of the spermatozoon is almost entirely made up of its nucleus, derived by repeated division from the nucleus of the primitive ovum, we shall see that in the fusion of female and male pronuclei we have really the fusion of a portion of the nucleus of a permanent ovum, which is itself derived from the nucleus of a primitive ovum, with the nucleus of a spermatozoon which is also derived by fission from and is therefore a part of a nucleus of a primitive ovum; i.e., that the fusion is between two parts of very similar morphological value.
If we inquire further as to the cause of this process of impregnation, the answer is probably to be found in the great advantage as to, vigor of the progeny which is known to accrue to both animals and plants from cross-fertilization as contrasted with self-fertilization; it may even be, as suggested by Balfour, that the habit of forming polar bodies—i.e., of providing that development cannot possibly occur without impregnation, has been acquired and perpetuated for the express purpose of insuring that cross-fertilization should be the invariable rule.
As to the number of spermatozoa necessary to insure fertilization, or normally taking part in it, our knowledge is very imperfect. From ob servations on the lower animals it would appear that a single spermato zoon is sufficient, but that more than one may be concerned in the act.