Marsupials may be defined as viviparous (that is non-egglaying) mammals, in which the young are born in an imperfect condition, and almost immediately attached to the teats of the mammary glands; the latter being generally enclosed in a pouch, and the front edge of the pelvis being always furnished with epipubic or "marsupial" bones.
Their abdominal bones are like those of the marsupials; and they are furnished with pouches for their young, but have no teats, the milk being distilled into their pouches from the mammary glands.
Examples of physiological hypertrophy are found in the ovaries, uterus and mammary glands, where there is an increased functional activity required at the period of gestation.
In the mammary gland during lactation or in sebaceous glands, caused by increased functional activity.
During the early period of their sojourn in the pouch, the blind, naked, helpless young creatures (which in the great kangaroo scarcely exceed an inch in length) are attached by their mouths to the nipple of the mother, and are fed by milk injected into their stomach by the contraction of the muscle covering the mammary gland.
Etymologically, however, that designation cannot be justified; for it is of hybrid (Latin and Greek) origin, and is equivalent to " mastology," the science which deals with the mammary gland (Gr.
Mammary Gland >>
In a younger stage of their development, however, the young are carried in a temporary abdominal pouch, to which they are transferred after hatching, and into which open the mammary glands.
The secretion of milk, if occurring in the mammary gland, is much diminished or entirely arrested.