Gestation

Gestation, the retention of the mammalian embryo in the uterus. The period of gestation—i.e. between the fertilisation of the ovum and the extrusion of the foetus—varies greatly, from about 18 days in the opossum and 30 in the rabbit to about 280 in man and 600 in the elephant. Robert Chambers in his Vestiges of Creation emphasised the importance of prolonged gestation as a factor of evolution, and it is certain that the more highly evolved mammals have longer periods of pregnancy than the lower. The size of the animal, the number of offspring at a birth, and the degree of their maturity at birth have also to be considered: thus, the gestations of cow and sheep last about 280 and 150 days respectively, those of mare and bitch about 350 and 60 days, those of giraffe and kangaroo about 420 and 40 days respectively. In the Marsupials, where the placental union between mother and offspring is still undeveloped, the birth is almost always very precocious, but in most cases the young are stowed away after birth in the external pouch. The lowest mammals—duckmole and Echidna—are oviparous. See FÆTUS, MAMMALS, PLACENTA, PREGNANCY, REPRODUCTION.