Mammalian ontogeny and development can, from a physiological standpoint, be separated into three general strategies. First, the monotremes such as platypus and echidnas are oviparous, meaning that they conceive young via copulation, but give birth to young inside an eggshell. After a short period of development in the egg, young hatch and then suckle the mother's milk as it leaks into the fur and not through a nipple as monotremes do not have nipples.
In contrast, marsupials and placental mammals are viviparous, meaning they give birth to live young. But their respective strategies differ, especially with regards to level of development of the young at birth. Marsupials give birth to live young, but they have a short gestation and produce young that are extremely altricial, i.e., very early in their development stage. Marsupial offspring are born blind and naked, and with underdeveloped organs except for a pair of extremely well-developed and strong front limbs. The young are also minuscule in size compared to adults. They spend most of their growth phase outside of the female reproductive tract but securely attached to the maternal nipple for nourishment, often, but not always, in a pouch. Kangaroos for example give birth to tiny young which then crawl to the pouch (with the strong front limbs) and find a nipple. Attachment to the nipple ensures that the blind and naked young does not fall off the mother and always remains close to its source of nourishment.
Placental mammals constitute the largest group of mammals. In these species, which includes, cats, dogs, horses, bats, rats and humans, fertilized ova migrate to the uterus where they implant and fuse with the lining of the uterus called en-dometrium, which then leads to the creation of a placenta, a highly vascular membrane that acts as the exchange barrier between embryo(s) and mother. Young develop inside the female tract to varying degrees, but even the most altricial of placental mammals (polar bears Ursus maritimus, for example) still are more developed at birth than marsupials. Internal development can be extremely advanced and lead to birth of young that are able to stand and run almost immediately after birth. Wildebeests, elephants and guinea pigs all have precocial young (offspring born fully developed) in this category.
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