Echidnas and platypuses are the only surviving monotremes, mammals that lay eggs. The echidna's egg is soft-shelled and roughly the size of a grape. About three weeks after mating the female curls up and passes the egg into an abdominal fold that has swollen into a functional pouch, where it incubates against her skin.
The hatchling, called a puggle, is naked, blind and about the size of a jellybean. It has no nipple to attach to: monotremes have no teats, and milk is secreted through patches of specialised skin called areolae, which the young laps from the surrounding fur. It stays in the pouch until its spines start to develop at around two months, and is then left in a nursery burrow and visited to be fed for several more months.
Echidna milk is unusual in another way. It contains a protein, since named EchAMP, with antibacterial properties, which may compensate for delivering milk onto exposed skin rather than through a sterile duct.