In the female mammal the times for sexual intercourse, instead of extending continuously over a season of considerable duration, as with the male, are restricted to periods of "heat" or oestrus. These may recur at rhythmical intervals within one breeding season (mare, cow, ewe, sow) or there may be only one oestrus to the season (bitch). The former condition has been described by Heape as polyoestrous, the latter as monoestrous. The whole cycle of changes is known as the oestrous cycle. In the case of a typical monoestrous mammal, such as the dog, the oestrous cycle is divided as follows: anoestrum (period of rest) ; prooestrum (period of growth and preparation) ; oestrus (period of desire) ; pregnancy or (alternatively) pseudo-pregnancy.
During the anoestrum the reproductive system is, relatively speaking, quiescent. The Graafian follieles which contain the ova probably undergo slow growth and ripening, but they do not become conspicuous upon the ovarian surface until near the end of the anoestrum. The uterus is relatively anaemic and the glands inactive. The mammary glands are also inactive unless lactation is in progress after recent pregnancy. The entire anoestrum in the bitch lasts about three months.
The prooestrum is marked by increased activity of the genera tive system generally. It is the time of "coming on heat." The follicles come to protrude visibly from the surface of the ovaries. The uterus also undergoes growth, the blood vessels increase in size and number, and the glands in the mucous membrane elab orate more secretion. At a slightly later stage a definite haemor rhage occurs in the uterus and blood is passed out to the exterior at the vulva. The mammary glands may also become slightly congested. The entire prooestrum lasts from one to two weeks and external bleeding may go on for ten days, but it is usually slight, consisting of no more than a sanguineo-mucous flow.
Oestrus or "heat" is the period at which (and, ordinarily, only at which) sexual intercourse takes place. It is marked internally by ovulation, that is, the rupture of the Graafian follicles and the discharge of the ova, which then become mature and ready for fertilization by spermatozoa. The wall of the uterus undergoes repair at this time but the glandular secretion is abundant and more liquid in character, to provide a suitable medium for the spermatozoa. In the bitch oestrus lasts about a week.
Oestrus is succeeded by either pregnancy or pseudo-pregnancy. Each of these periods in the bitch lasts about two months. At their termination the uterus and the generative organs pass back to a condition of rest, and so the oestrous cycle is repeated. The
complete cycle takes about six months in the bitch, there being typically two cycles and two oestrous periods in the year but there is a good deal of individual and racial variation.
If pregnancy takes place as a result of fertilization of the ova, discharged during oestrus, these segment and become attached to the inside wall (mucous membrane) of the uterus, which grows around each of them. The structure formed in this way is highly vascular and serves as the organ of nourishment for the develop ing embryos to which the ova give rise. (See VERTEBRATE EM BRYOLOGY.) This organ is the placenta and is characteristic of nearly all mammals. The embryos are attached to the placenta of the mother by the outer of a number of membranes, and vas cular processes (villi) grow out from this membrane (the chorion) into the hypertrophied uterine mucous membrane now forming the maternal placenta. Thus a close connection is formed between the embryo and the mother and the placenta acts as an organ of respiration, supplying the developing young with oxygen brought thither in the maternal blood, and an organ of excretion, getting rid of carbon dioxide and the waste nitrogenous products, besides supplying the necessary nutriment. During pregnancy also the uterine muscles undergo a great hypertrophy, and are responsible for a great part of the increased weight which occurs in that organ. Thus, in the human subject, the virgin uterus weighs about 3o grams, whereas the same organ at the close of pregnancy, apart from the contained young, weighs i,000 grams. It is through the rhythmical contraction of the uterine muscles that the young are expelled in parturition (the act of giving birth). The mammary glands undergo great development during pregnancy in preparation for the secretion of milk at its close.
If the ova discharged at ovulation are not fertilized during oestrus (as when coition does not occur) they die in the uterus and disintegrate. Nevertheless, in the bitch and many other mammals the uterus and mammary glands pass through growth changes which, though not so pronounced, are similar in character to those during pregnancy. Thus, the mucous membrane be comes highly vascular and the glands greatly enlarge. The mam mary glands also undergo marked development and towards the end of the period secrete milk. Even virgin bitches secrete milk freely about two months after the cessation of oestrus. At the end of this pseudo-pregnancy the generative system as a whole subsides into a condition of rest.