Description : Do all mammals have a placenta?
Last Answer : Mammals of the monotreme group (platypus, echidnas) are oviparous, egglaying, and they do not have a placenta. Mammals of the marsupial group (kangaroos, koalas, opossums) do not have a ... mother's pouch. Placenta only forms in female placental mammals. Mammalian Review - Image Diversity: placenta