09/02/2017
There are three sub classes of mammals, placental mammals which carry their young in a womb with a placenta, and deliver a relatively full-formed infant, marsupials which do not have a placenta and deliver a relatively un-formed infant that continues its development inside an external pouch, nourishing itself with milk from the mothers mammary glands, and monotremes which lay eggs.(check them out).
The majority of animals native to Australia are marsupials, but what exactly is a marsupial, and how do they differ from other animals?
Simply put, the embryo of a marsupial starts out life in the mothers womb, receiving nourishment from its egg sac, but is delivered very early and continues its development outside the mothers body.
Instead of having a womb lined with a placenta that the egg attaches itself to and gains nourishment from, marsupials have no placenta and the baby must leave the womb, while still in a nearly embryonic state, to get nourishment from breast milk.
The baby, called a "neonate" at this stage, is born after only 12 to 28 days of development in the womb. It may be a fraction of an inch long in some species, is hairless, blind and has poorly developed rear legs.
The nostrils are open and the fully developed sense of smell is critical to the neonate finding its way. Its forelimbs are strong too, and raging instinct sets the neonate on a manic climb, unaided by the mother, across the sea of its mothers belly fur to reach the pouch.
This journey may be a distance of as much as 15 inches and require six minutes of continuous struggle. In a miracle of pure instinct, the almost imperceptible newborn arrives at the pouch, where it will attach itself to a teat and continue to develop for another 2 to 6 months.