The mammals can be differentiated into three main groups on the basis of the development of their babies. These three groups are marsupials, monotremes, and placental mammals, which is the largest group. The monotremes refer to the mammals, which lay eggs. The marsupials refer to the mammals, which give birth to young ones that are not developed completely. While in a placental mammal, the development takes place within the body of a mother until and unless its systems of the body start to work on their own.
