After birth the development of higher mammals is prolonged over the period corresponding in man to infancy, childhood and adolescence ; the delayed maturity affording opportunity for long-continued growth and differentiation of the brain, with a correspondingly extended period of learning.
In wide contrast with this are the conditions found in the lowest existing mammals, the egg-laying mammals of Australia (see MONOTREMATA) While these lay eggs, like reptiles, the embryo nevertheless derives some of its nourishment from the uterine wall as well as from the yolk stored in the egg.
In typical placental mammals the right and left uteri are fused in the mid-line, making a single uterus. In the lowest mammals, the monotremes, the right and left uteri are entirely separate as in reptiles. Various intergrading conditions between these ex tremes occur in the marsupials, edentates and rodents.
The milk-glands of female mammals are highly characteristic of the whole class. They are arranged in pairs on the ventral surface of the body, varying in number from one pair in man and other primates to eleven pairs in the insectivore Centetes. They
are usually surmounted by nipples, raised folds of skin with a central tunnel for the passage of the milk. In the placental mammals the milk is actively sucked or pumped by the young but in the monotremes the nipples are represented only by de pressed glandular areas with raised borders and the milk is said to be licked by the young from the base of the hairs in the mammary field. This condition may well give the clue to the origin of the milking habit, which must have originated after the sebaceous glands of the skin had developed an albuminous secre tion, which may have served originally either to attach the eggs to the under-surface of the brooding mother, or to anoint the eggs with a heat-retaining coating. At any rate, the habit of milk ing in its initial stages seems to require some voluntary action by the young, such as licking.