Thefimbrioe, or finger like processes of the Fallopian tube, grasp the ovary at the point where the rupture of the ovisac, or Graafian follicle, is about to occur, and when the ovum is received into one of the tubes it is passed along toward the uterus, partly by the current established by the ciliated epithelium lining the tube, and partly by the peristaltic movements of the ducts on either side.
The ovum is probably almost always impregnated by a spermatozoid not in the uterus, but in the distal, or outer part of one of the Fallopian tubes, one-third to one half way down from the fimbriated extremity to the uterus, it being remembered that the spermatozoa reach these parts of the Fallopian tubes by their own independent active movements.
In the ovum discharged once in every lunar month certain important changes occur independently of impregnation, so that each ripe ovum is prepared to meet a spermatozoid whether the latter be there or not.
The germinal vesicle moves to the surface of the ovum, disappears from view, and in its place two polar globules appear, while a portion of the original germinal vesicle moves back toward the centre of the ovum to form the female pronucleus. The object of the polar, or directing globules, is to facilitate the entrance of a spermatozoid, while the female pronucleus is an indication that the ovum is ready for impregnation.
The female pronucleus is thus seen to be a part of the original germinal vesicle, and it is now adapted to blend itself with the head of a spermatozoid, which, if it should happen to fuse with the ovum, buries its head and intermediate segment in the yolk substance so as to constitute the germ of an embryo.
Especially bear in mind that these changes, which result in the formation of the female pronucleus, occur in the ovum, whether it be impregnated or not.
If unimpregnated it passes off with the menstrual discharges; but if fertilized it plants itself on the mucous membrane of the uterus to develop myriads of cells, which become differentiated and specialized in the fabric of a human being.
Out of the tens of thousands of spermatozoa which may have found their way up to the distal extremities of the Fallopian tubes, only one is concerned in the process of fecundation, the other less fortunate ones becoming no more than refuse. All have been trying, as it were, to force their heads through the egg-pod, or zona peUucida, of the ovum, but only the one succeeds.
The fecundating part of each spermatozoid is its head and intermediate segment, the tail being designed solely as a locomotor apparatus to propel it to its destination and enable it to penetrate the walls of the ovum. This zona pellucida of the ovum which the spermatozoon is required to penetrate is analogous to the thin white "skin" enveloping a hen's egg, which is readily seen by cracking off the shell from a hard boiled egg, but, of course, in the human ovum it is much more delicate.
The ripe ovum having prepared itself for impregnation by the formation of the female pronucleus and the polar or directing globule, the head of the favored spermatozoid here finds a spot in the cell wall of the ovum which has been thinned out and weakened, and thus it is forced within by the lashing movements of its tail.
This conical projection of the ovum, the polar or directing globule, after the head and intermediate segment of the spermatozoon have entered, contracts and cuts off the latter's tail, which, having performed its function, is now useless.