The Opossums: The pouch
The earliest mammals were probably like the opossums that today live in the Americas particularly those belonging to the genus Didelphis. The Virginiaopossum(Didelphismarsupialis)of North America is a large rat shaped creature, with small eyes and a long naked tail which it can wrap round a branch with sufficient strength to support its own weight. It has a large mouth that opens wide and is equipped with a great number of small sharp teeth. It is a tough adaptable creature that has spread through the Americas, from Argentina in the south to Canada in the north. One of the most extraordinary aspects of this animal is its manner of reproduction. The female has a capacious pouch on her underside in which she rears her young. The young are extremely small and without fur and have attach themselves to the mother's teats. The method by which they get there is one of the most fascinating. The opossumscopulate and fertilization of the female's eggs occurs internally. The young embryos, however, have only enough yolk to maintain themselves for the first few days of their life. At twelve days and eighteen hours the animals are expelled into the outside world. This represents the shortest gestation period known for any mammalspecies. These young are born so premature that they are no larger than bees, and so unformed that they are not called infants but are rather referred to as neonates. As the neonates emerge from their mothers cloaca, they haul themselves through the fur of her belly to the opening the pouch, a distance of some 80 mm. Only about half of the neonates reach the pouch and each animals attaches itself to one of thirteen nipples and starts to take milk. If more than thirteen complete the journey, only those that attach themselves to a teat will survive. Nine or ten weeks later, the young clamber out of the pouch. They are now fully formed, the size of mice, and cling to their mother's fur. In about three months they leave their mother for an independent life of their own. Mammals that bread in this way (by means of a pouch) are all placed in the order Marsupialia.