. Principles of modern biology. Biology. 386 - Multicellular Animals, Especially Man permeated with blood sinuses, and ordinarily, when the sinuses are not distended with blood, the tissue is flaccid. But one of the reflexes in the pattern of sexual excitation leads to a constriction of the vessels that drain the sinuses. At this time, therefore, the erectile tissue becomes turgid and distended with blood; and temporarily the penis be- comes an effective intromittent organ. When ejaculation occurs, the urethra is swept by a short but powerful series of peristaltic con- tractions. THE FEMALE RE
. Principles of modern biology. Biology. 386 - Multicellular Animals, Especially Man permeated with blood sinuses, and ordinarily, when the sinuses are not distended with blood, the tissue is flaccid. But one of the reflexes in the pattern of sexual excitation leads to a constriction of the vessels that drain the sinuses. At this time, therefore, the erectile tissue becomes turgid and distended with blood; and temporarily the penis be- comes an effective intromittent organ. When ejaculation occurs, the urethra is swept by a short but powerful series of peristaltic con- tractions. THE FEMALE REPRODUCTIVE ORGANS The ovaries, in vertebrate animals, are not connected directly with the oviducts. In the frog, for example, each large ovary or egg mass, lies in the bod)' cavity, ventral to the corresponding kidney (Fig. 21-4). The ostium, or dilated mouth of the oviduct, opens di- rectly into the body cavity in a region some- what anterior to the ovary (Fig. 21-4). To be liberated, therefore, the eggs must break through the wall of the ovary and pass through the body cavity to the ostium. This rupturing of the ovarian wall is referred to as ovulation, and after ovulation the eggs are carried to the mouth of the oviduct by the activity of the ciliated epithelium that lines the peritoneal cavity. The eggs of the frog do not possess any external coating of "jelly" before they enter the oviduct; but while an egg is passing through the oviduct it receives its "jelly coat" from gland cells lining the wall. Like the sperm duct, the oviduct of the frog leads to the cloaca rather than to the exterior, and the eggs are finally voided through the cloacal opening. Just before joining the cloaca, the oviduct shows a slightly expanded section, called the "uterus" (Fig. 21-4). However, no development occurs while the eggs remain in this part of the oviduct, and consequently the term uterus is not a very apt one. True copulation does not occur in the frog, but
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