What is the difference between the development of embryos in placentals and marsupials?

The embryo in a placental mammal is fully developed within the mother's uterus, sustained by the placenta, through which it receives all the nutrition it needs. Its organs are fully developed and it is able to survive outside the mother's body once it is born. It requires mothers' milk (or a suitable substitute) when it is born, but it can live independently of the mother, given the right care.

Baby marsupials are born extremely undeveloped, and are unable to survive at all outside the mother's body, until they can reach the pouch. They are born after a much shorter gestation period than a placental of equivalent size, and even the largest species of marsupial is only the size of a jellybean when it is born. After birth, it must climb up the outer fur of the mother and into the pouch where it attaches itself to a nipple. The teat swells in the joey's mouth, securing it in place so it cannot be accidentally dislodged. The joey will usually stay in the pouch for about 6-9 months, depending on the species, until it is developed enough to leave the pouch and move about independently.