GESTATION, in physiology, is the term applied to the period that intervenes in the inammalia between impregnation and the bringing forth of the young. The period and the number of young produced at a birth vary extremely in different mammals, but usually stand in an inverse ratio to one another. Thus, in the largerherbivora, as, for example, the elephant, the horse, the ox, and the camel, the female seldom produces more than one at a time, but the period of gestation is long; while.in the smaller ones the progeny is numerous, but the period of gestation only a few weeks. In the elephant, the period of gestation extends over twenty or twenty-one months; in the giraffe, it is fourteen months; in the dromedary, it is twelve months; in the mare, upwards of eleven months; in the tapir, between ten and eleven; in the cow, nine; and in many of the larger deer somewhat more than eight months. In the sheep and goat, the period is five months. In the sow, which produces a numerous litter, the period is four months. In the rodentia, the progeny is numerous and imperfectly developed, and the period of gestation is comparatively short: in the beaver, one of the largest of the order, it is four months; in the rabbit and hare, from thirty to forty days; in the dormouse, thirty-one days; in the squirrel and rat, four weeks; and in the guinea-pig, three weeks or less. The
young of the carnivora, like the yowlg of the rodentia, are born with their eyes closed, and in a very immature condition; and in even the larger carnivora the period of gesta tion is far shorter than in the larger ruminantia or pachydermata; it is six months in the bear; one hundred and eight days in the lion (the period in this animal is stated by Van der Hoeven at three months); seventy-nine days in the puma; sixty-two or sixty-three days in the dog, the wolf, and the fox; and fifty-five or fifty-six days in the cat. In the marsupial animals, which, from a structural peculiarity, produce their young in a far more immature state than any other mammals, the period of gestation is -eery short, being thirty-nine days in the kangaroo, the largest of the marsupial animals, and only days in the opossum. Nothing certain is known regarding the period of ges tation of the cetae'ea. The quadrumana produce one, sometimes two, at a birth; and the period of gestation, as far as has been observed, seems to be seven months. In the human race, forty weeks is the usual period of gestation, but this period is liable to certain deviations, which are noticed in the article FICETIIS.