ABSTRACT
Monotremes, like their reptilian ancestors, lay eggs, but only about 10 days after fertilization: early embryonic development up to the fetal stage takes place inside the mother. Once the small quantities of yolk have been absorbed, the embryo gets its nutrition from maternal secretions into the oviduct that pass through its parchment-like shell (Cruz, 1997; Renfree, 2010). After the eggs are laid, the mother keeps them warm for a further 10 or so days until they hatch at about the same state of development as those of newborn Metatheria. Monotremes lack nipples: their mammary glands secrete milk directly to the skin where the young puggles can lick it. Echidnas have a pouch for rearing their young, so providing protection for embryos; platypuses lack a pouch and their young have to cling to their mothers.
