The development of the various parts of the mammary gland will be discussed in the following order: Mammary streak, mammary line, mammary hillocks, milk ducts, formation of lumen, new-born secretion, puberty changes, changes from puberty to the first pregnancy, the stroma, the nipple, supernumerary mammary glands, effects of inanition on the developing mammary glands and comparison of the developing mammary glands in the two sexes.
Mammary Streak, Mammary Line and Mammary Hillocks.
—The mammary gland makes its first appearance quite early in the embryonic life of various species of animals. In the albino rat Henneberg1 observed the first trace of the mammary gland in embryos of eleven days of gestation. Lustig2 described the mammary gland as first appearing in human embryos of 4.75 mm. in length. The mammary gland in each species above mentioned is first represented on each side of the embryo by a faint light streak