Live young

Among animals, viviparity is development of the embryo inside the body of the parent, eventually leading to live birth, as opposed to reproduction by laying eggs that complete their incubation outside the parental body.

Viviparity and the adjective viviparous derive from Latin vivus ("living") and parire ("to bear young").

Five modes of reproduction have been differentiated in animals based on relations between zygote and parents. The five include two nonviviparous modes: ovuliparity, with external fertilisation, and oviparity, with internal fertilisation. In the latter, the female lays zygotes as eggs with a large yolk; this occurs in all birds, most reptiles, and some fishes. These modes are distinguished from viviparity, which covers all the modes that result in live birth:

At least some transport of nutrients from mother to embryo appears to be common to all viviparous species, but those with fully developed placentas such as found in the Theria, some skinks, and some fish can rely on the placenta for transfer of all necessary nutrients to the offspring and for removal of all the metabolic wastes as well once it has been fully established during the early phases of a pregnancy. In such species, there is direct, intimate contact between maternal and embryonic tissue, though there also is a placental barrier to control or prevent uncontrolled exchange and the transfer of pathogens.

In at least one species of skink in the large genus Trachylepis, placental transport accounts for nearly all of the provisioning of nutrients to the embryos before birth. 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. The embryo then produces invasive chorionic tissues that grow between the cells of the uterine lining till they can absorb nutrients from maternal blood vessels. As it penetrates the lining, the embryonic tissue grows aggressively till it forms sheets of tissue beneath the uterine epithelium. They eventually strip it away and replace it, making direct contact with maternal capillaries. In several respects, the phenomenon is of considerable importance in theoretical zoology. The authors remark that such an endotheliochorial placenta is fundamentally different from that of any known viviparous reptile.

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