The most famous feature of monotremes is that they lay eggs. This is not to say that
the embryo is not nourished in the uterus and they have an allantoic and yolk sac
placenta (Griffiths 1968, 1978, Tyndale -Biscoe and Renfree 1987). The egg is laid
when the embryo has reached the 19-20 somite stage, and in the echidna, but not the
platypus, a pouch develops to receive the egg (Griffiths 1978, Renfree 1993). The
young is dependent on milk for the first few months of life to sustain its development.
Like all other mammals, the milk is produced by well-developed mammary glands.
Unlike all other mammals, the females have no nipples and the milk is expressed by
the female from two teatless areolae and the young suck the milk directly from the
abdominal surface (Griffiths 1968, 1978). Monotreme milk composition, like that of
marsupials, changes throughout lactation and the composition of platypus and echidna
milks differ significantly.