The ovaries, the essential reproductive organs of the female, likewise serve a double function. They produce the ova and also elaborate internal secretions comparable to those of the testes; these secretions are responsible for initiating the development of the female characters, as well as being a necessary factor in the sexual and reproductive processes. In the lower vertebrates the ova are large owing to the amount of food substances (yolk) contained in them (as with the egg of the fowl), but in mammals they are microscopic, each being about a 0 in. in diameter (this, however, is considerably bigger than a spermatozoon, which is about in. in length). The ova are contained within little sacs (Graafian follicles). These begin by being very small, but as they approach maturity their cavities enlarge until they pro trude from the surface of the ovary ; eventually (unless, as often happens with some of them, they have degenerated) they dis charge their ova to the exterior in ovulation.
The ovaries are attached, one on either side, to the dorsal wall of the abdominal cavity by the broad ligament. The tubes which convey the ova to the exterior are also suspended by this ligament, a double fold of tissue arising from the wall of the body cavity. In the lower vertebrates the oviducts are provided with glands secreting albumen or egg-white which coats the ovum as it passes down the tube. The egg-shell in those animals in which it is formed is also secreted by a gland ; in birds this is at the posterior end of the oviduct just in front of where it opens into the common urogenital passage. At the anterior end each oviduct has a fimbriated trumpet-shaped aperture which expands at ovulation and receives the eggs as they pass into the body cavity. The interior of the expanded end is provided with cilia which direct the passage of the ova into and down the tubes. The oviducts are usually paired to correspond with the ovaries, but in birds only the left ovary and oviduct are present. In mammals and birds the ova are usually fertilized by the spermatozoa in the passage of the oviduct, but in the lower vertebrates (e.g., most fish) this often occurs outside the body. In mammals, the ovi ducts (small somewhat coiled tubes) swell out posteriorly to form the cornea uteri, or womb. These may continue double through out their entire length and open separately into the vagina (rabbit) or after continuing separate for a considerable distance may unite together to form the corpus uteri or body of the uterus (cow, sheep, mare, bitch, etc.) or they may extend for only a short distance before opening into the corpus uteri, which is a sac or bag (man). At the hind end the corpus uteri narrows down to form a neck (cervix) and this opens into the vagina by the os uteri. The uterus is the organ which contains the develop
ing young during pregnancy. It has thick muscular walls on the Outside and a mucous membrane with numerous glands lining the cavity inside. These secrete a fluid which helps to nourish the developing embryo during pregnancy and supplies a medium in which the spermatozoa swim after copulation. The vagina is the broad urogenital passage which extends backwards through the pelvis and opens to the exterior at the vulva. The latter is consti tuted by all the female generative organs visible externally. The lateral boundaries are the labia or lips. The clitoris is a small rod-like erectile structure and corresponds to the penis of the male but is solid.
The mammary glands, although not directly concerned with the reproductive processes, are dependent upon the ovaries for growth. They consist of milk-secreting tissue and are provided with ducts which convey the milk to the nipple, whence it can be drawn off.
The male breeding season, when it occurs, is called the season of rut. The increase in the size of the testes which occurs prior to rut is accompanied by activity not only of the cells which give rise to the spermatozoa (the spermatogenetic tissue of the sem iniferous tubules) but also of the interstitial cells. In some mammals the testes are not permanently retained in the scrotum but descend thither at the beginning of rut and are withdrawn into the abdomen again after the rutting season is over (e.g., many rodents). In insectivores (e.g., mole and hedgehog) the testes descend periodically into temporary receptacles. In the mole it is estimated that the testes increase in size 64 times, and the seminal vesicles, prostate and other accessory glands like wise show enormous growth. The time for sexual intercourse is continuous throughout rut, there being no short periods of quiescence within the breeding season as in the females of many species. Amongst domestic animals generally there is no special season of rut, the male being capable of service throughout the year, the semen evacuated normally containing an abundance of spermatozoa. In this respect these species differ from their wild ancestors, for in the undomesticated state the male usually ex periences a rutting season at the same time as the breeding season in the female.