Mammals develop their young within the body of the maternal organism. Young mammals, like young birds, begin their development in an egg, but the mammalian egg is too minute to afford nourishment beyond the first few hours of development. Between that time and the birth of the young the nourishment is derived from the blood of the maternal organism, and consists of proteins, sugars, and salts absorbed from the walls of the uterus by the growing young. After the birth of a young mammal there is a wonderful provision for its nourishment that deserves more than passing attention. Nature provides for young mammals a wonderful food called milk. This is produced by lactiferous or milk-producing glands on the ventral surface of the mother's body. Within a few hours after the birth of a young mammal, it begins to draw milk from these glands into its stomach, where it is digested and assimilated, and built up into muscle, gland, nerve, bone, etc.

Milk on chemical analysis is shown to consist of proteins, sugar, fat, salts, and water. From these substances all the tissues of the growing animal are constructed; all repairs are made, and fuel is furnished for the body movements and for heat.