In the uterus, the eggs are very small, about 1mm in diameter, with very little yolk and very thin shells.The shell membrane is vestigial and transient; its disintegration permits the absorption of nutrients from uterine secretions.
In some trees, like Jackfruit, some citrus, and avocado, the seeds can be found already germinated while the fruit goes overripe; strictly speaking this condition cannot be described as vivipary, but the moist and humid conditions provided by the fruit mimic a wet soil that encourages germination.
However, the seeds also can germinate under moist soil.
Since the developing offspring remains within the mother’s body, she becomes, in essence, a walking incubator, protecting the developing young from excessive heat, cold, drought, or flood.
This offers powerful options for dealing with excessive changes in climate or when migration events expose populations to unfavourable temperatures or humidities.
One traditional hypothesis concerning the sequence of evolutionary steps leading to viviparity is a linear model.
According to such a model, provided that fertilization was internal, the egg might have been retained for progressively longer periods in the reproductive tract of the mother.
Genotypic sex determination is also found in most reptiles, including many viviparous ones (such as Pseudemoia entrecasteauxii), whilst temperature dependent sex determination is found in some viviparous species, such as the montane water skink (Eulamprus tympanum).
Viviparous plants produce seeds or embryos that begin to develop before they detach from the parent.
In the latter, the female lays zygotes as eggs with a large yolk; this occurs in all birds, most reptiles, and some fishes.