But perhaps the most conspicuous difference between marsupials and placentals is in the degree of development of the young at birth. A placenta is formed in only a few species, and even in those the gestation period is extraordinarily short. At the time newborn marsupials emerge from their mother's reproductive tract and crawl to the pouch, they are tiny and have just begun forming functional organs. The heart, kidneys, and lungs are all barely functional. Even the brain is at a very early ontogenetic stage.
Most development takes place in the pouch, and the lactation period is prolonged.
Lillegraven argues that marsupial young must be born quickly, before the mother's immune system can respond the presence of foreign tissue in the form of a developing embryo. Most development takes place in the pouch, safe from maternal immune attack. Radiations took place on both of these continents during the Cenozoic, at a time when there were few placental competitors. But past marsupial faunas were even more incredible. It seems clear on both continents that invasion by placentals is correlated with a decline in number and diversity of marsupials.