Final answer:
Marsupials give birth to undeveloped embryos that continue their growth and development in the mother's pouch. This form of development reduces risks to the mother but makes the newborn more vulnerable compared to placental mammals.
Step-by-step explanation:
A marsupial completes its embryonic development externally after a very short period in the uterus. Unlike placental mammals, marsupials give birth to a tiny, immature embryo that is nourished initially by a yolk sac. Soon after birth, the embryo moves to the mother’s pouch and clings to a nipple, where it continues to grow and develop for several months. This mode of reproduction is seen in Australian species such as kangaroos, koalas, and the Tasmanian devil, as well as American opossums.
Marsupial development has its advantages, one of which is reducing the risk to the mother during gestation. The mother does not need to carry a large fetus internally for an extended period, avoiding potential immune system conflicts and the nutritional burden of supporting a developing fetus. However, as the embryo is born very small and fragile, its chances of survival may be lower than those of placental mammal offspring who have longer gestation periods.