Where does a young marsupial develop?

Young ones of marsupials(joey) are born in very immature condition. Thereafter they develop inside marsupial pouch remaining attached to mammary teat of mother.

Marsupial joeys are born in very immature condition, they resemble a fetus at the time of birth. This is because their placenta is short lived. After coming out of mother's womb the joey climbs slowly towards mother's pouch. There the joey clings to a mammary gland outlet by mouth to suck nutritious milk. This is why such young joey is called a mammary foetus .

Joey emerges from mother's marsupium after attaining some maturity.