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The biological significance of the process in humans can best be explained by reference to the reproductive function in other mammals. In a number of species of wild sheep, for example, there is only one breeding season in the year; during this season a cycle of changes takes place in the reproductive organs,characterized by ripening and release of ova from the ovaries, increased blood supply to the genital tract, growth of the uterus, and proliferation of its lining.

There is a discharge of blood and mucus from the uterus and vagina, and this is the time when coition may take place. Pregnancy normally follows, but if the ewe is not served by the ram the changes retrogress until the next breeding season. This cycle of changes is termed theestrous cycle.

In many domesticated sheep there is more than one estrous cycle in the breeding season. If the ewe does not become pregnant in the first cycle there is a short resting phase; then ovulation is repeated and another cycle of activity of the reproductive system takes place. After each breeding period, with its succession of estrous cycles, there is a relatively long resting phase.

In most female primates, including women,there is no resting phase; an unbroken series of estrous cycles occurs throughout the year, and pregnancy can occur in any one of them.

In some animals a variety of external stimuli act through the central nervous system on the hypothalamic region of the brain.The hypothalamuscontrols the release from thepituitary glandof hormones that induce ripening ofovarian follicles—ova and the cellular structures that enclose them. These pituitary hormones,called gonadotropic hormones, are carried to the ovaries by way of the bloodstream.

In primates the hypothalamic mechanism normally is independent of external stimuli, and regular discharge of ova into the tubes leading to the uterus occurs even in the absence of coitus. Under the influence of the pituitary gonadotropic hormones, the ovary produces other hormones, which cause growth and increased vascularity of the uterus and vagina. These hormones areestrogens—chiefly 17 beta-estradiol—andprogesterone. It is as though the ovary prepares the uterus for the reception of the ovum that is released in the particular cycle.

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