<P> Mammalian ova contain only a tiny amount of the nutritive yolk, for nourishing the embryo in the early stages of its development only . In contrast, bird eggs contain enough to supply the chick with nutriment throughout the whole period of incubation . </P> <P> In the oviparous animals (all birds, most fish, amphibians and reptiles) the ova develop protective layers and pass through the oviduct to the outside of the body . They are fertilized by male sperm either inside the female body (as in birds), or outside (as in many fish). After fertilization, an embryo develops, nourished by nutrients contained in the egg . It then hatches from the egg, outside the mother's body . See egg for a discussion of eggs of oviparous animals . </P> <P> The egg cell's cytoplasm and mitochondria are the sole means the egg is able to reproduce by mitosis and eventually form a blastocyst after fertilization . </P> <P> There is an intermediate form, the ovoviviparous animals: the embryo develops within and is nourished by an egg as in the oviparous case, but then it hatches inside the mother's body shortly before birth, or just after the egg leaves the mother's body . Some fish, reptiles and many invertebrates use this technique . </P>

When are the eggs (ova) formed in a woman