This lack of efficiency is accommodated by reliance on a food that doesn't need to be chewed, milk. Evidence from the fossil record suggests that the earliest mammals probably were diphyodont, whereas late therapsids probably maintained the ancestral pattern of replacing teeth more than once. Mammary glands are made up of a system of ducts surrounded by glandular tissue, which secretes milk. Mammary glands differ somewhat in form from species to species of mammals. Those of monotremes are simple aggregations of glandular tissue along the abdominal wall.
Milk is secreted into depressions and is licked off of the fur by the young.
Mammary glands also differ in location and number. Some marsupials have up to 19 or 20 nipples, located in a pouch. Many mammals have a single pair, but among these the location varies. It is a very rich food source, which varies considerably among species in composition. The ADW Team gratefully acknowledges their support.