Final answer:
Marsupials, such as kangaroos and koalas, produce young that are born immature and develop in the mother's pouch, contrasting with viviparous mammals where young develop inside the womb and are born at a more advanced stage.
Step-by-step explanation:
In marsupials, the young are born in an extremely immature state and complete development in their mother's external pouch. Marsupials, such as kangaroos, koalas, and opossums, are distinguished from other mammals by their unique reproductive process. The marsupial embryo is nourished by a yolk sac rather than a placenta and, after a very short gestational period, the underdeveloped newborn, or joey, must make its way to the mother's pouch. In the safety of the pouch, it attaches to a nipple and continues to grow and develop for several months.
The birth and development process in marsupials is different from viviparity, where most mammals, some cartilaginous fish, and a few reptiles give birth to live young that have developed inside the mother's body, often connected by a placenta. The marsupial method of reproduction is seen as less risky for the mother, as they don't have to carry a large fetus or consume as much food. Yet, the fragility of the tiny newborn marsupial means that it may be less likely to survive compared to more developed placental mammals born through viviparity.