Do all mammals have a placenta?
Mammals of the monotreme group (platypus, echidnas) are oviparous, egg- laying, and they do not have a placenta. Mammals of the marsupial group (kangaroos, koalas, opossums) do not have a placenta either; females of this group give birth to embryonic young that then continue development within the mother’s pouch. Placenta only forms in female placental mammals.
