The egg after its discharge arrives at the abdominal orifice of the Fallopian tube, which communicates directly with the abdominal cavity. Some authors state that the end of the tube becomes applied against the ovary by the aid of muscular movement and, so to speak, sucks in the discharged ovule, while others hold that the movements of the vibratile cilia, with which the epithelium of the tubes is furnished, suffice to draw the ovule into its cavity. Figure 18 explains this phenomenon.
Having arrived in the tube, the ovule moves very slowly in the almost capillary tube by means of the vibratile cilia and arrives in the cavity of the womb. Fecundation probably takes place most often at the entrance to the tube or in its canal; sometimes possibly in the womb. On some occasions a squad of spermatozoids advances to meet the descending egg, and numerous spermatozoids are often found in the tubes, even as far as the abdominal cavity.
Fixation of the egg. Formation of the Decidua.—After fecundation, the egg becomes attached to the mucous membrane of the cavity of the womb. This mucous membrane proliferates and becomes gradually detached from the womb to form the membrana decidua which envelops the egg or ovule. An egg fecundated and fixed in this way may keep its position and grow during the first weeks of pregnancy, by the aid of villosities covering its envelope which penetrate the wall of the womb.
Fig. 18. Diagrammatic section in median plane of the female genital organs. It shows the position of an ovule which has just been discharged lying in the opening of the right tube, and that of another ovary fecundated and surrounded by the decidual membrane. In reality this could hardly coexist with the other ovule freely discharged. In the right ovary are seen ovules in various degrees of maturity in their Graafian follicles: also a corpus luteum—an empty Graafian follicle after expulsion of the ovule. The figure also shows the end of the penis in the vagina at the moment of ejaculation of semen, and the position of a preventive to avoid fecundation.
Fig. 19. The mouth of the tube applied to the ovary at the moment of expulsion of the ovule.
The womb. The placenta. The womb or uterus is the size of a small egg flattened in one direction. It terminates below in the neck or cervix, which is prolonged into the vagina as a projection, called the vaginal portion of the uterus. The cavity of the womb is continued into the neck and opens below in the vagina by an aperture which is round in virgins and is called the external os uteri. The walls of the womb consist of a thick layer of unstriped muscle. When childbirth takes place it causes tearing which makes the external os uteri irregular and fissured. During copulation the aperture of the penis or male organ is placed nearly opposite the os uteri, which facilitates the entrance of spermatozoa into the uterus. (For the illustration of these points see Fig. 18.)
The vitellus and the membrane of the egg enlarge with the embryo and absorb by endosmosis the nutritive matter necessary for the latter, contained in the maternal blood. The womb itself enlarges at the same time as the embryo.