Viviparity Oviparous - animals lay eggs in the environment and their embryos develop outside the mother s body. Viviparous - animals retain the embryo within the mother s body during its early developmental stages. All other vertebrate groups are viviparous except the ones above and the crocodiles and turtles. There is a big difference between viviparity in mammals and viviparity in sharks. Mammals have a specialized portion of the female reproductive tract, the uterus, that holds and provides nutrients for the embryo.
Sharks simply retain the fertilized eggs in the mother s body until they hatch.
This is called ovoviviparity - embryos receive their nutrition from the stores that were in their egg. Even among mammals there are various degrees of uterine adaptation. In marsupials the uterus simply holds the embryo until the young crawls into a pouch and completes development in the pouch Eutherian mammals - mammals other than monotremes and marsupials. In most mammals, the testes are outside the body in a pouch of skin called the scrotum . Within the testes, sperm are produced in tightly coiled seminiferous tubules .