III.

Reproduction

All mammals reproduce sexually, and two types of reproductive acts are used to bring about sexual conjugation. In the primitive egg-laying mammals, excretory and genital organs open into a common orifice, called the cloaca. Transfer of sex cells from the male to the female is accomplished by bringing the cloacae next to each other. In all other mammals, however, the male sex cells are transmitted by copulation. After fertilization, development of offspring takes place entirely within the body of the mother in all mammals except the monotremes, which produce leathery-shelled eggs with large yolks, and in many marsupials, in which the gestation period lasts only about 10 to 15 days, with most of the development taking place in the mother's pouch after birth. Mammalian young are not sufficiently well developed to pursue an independent existence immediately following birth but must be nursed during infancy. See Reproduction.