In viviparous animals the zygote develops into a young one inside the body of the female organism. After attaining a certain stage of growth, the young ones are delivered out of the body of the female organism. Because of proper embryonic care and protection, the chances of survival of young ones is greater in viviparous organisms. Not all mammals are viviparous; some are egg-laying, i.e., oviparous e.g., duck-billed platypus.