Uglogical

Reptiles fertilization is scheduled differently with respect to most species it seems to occur, as might be expected, were laid; but in some the sperm may live on and continue to fertilize eggs months or even years has taken place. The longest known periods of such deferment is four years for the diamondback terrapin of the south years in the case of the tropical American cat-eye snake. It is significant of the three reptiles which venture farthest north, even across the Arctic Circle, two- the European viper and the lizard Lacerta vivipara – bear their young alive. So does the slow worm (Anguis), another venturer into northern regions. The cold ground of those areas, no doubt, is not well suited to incubating eggs. Neither is water, so far  as shelled eggs are concerned, which explains why most reptiles with strongly aquatic habits also bear their young alive.

Many of the live-bearing reptiles, however, belong to groups that have egg-laying members too. The skinks, the lacertas, the boids and the vipers are examples. There are even species that lay eggs in some parts of their ranges but bear live young in other parts. This suggests that their viviparity – as the ability to produce live young is called – is not so formal an undertaking as it is in mammals, and this is true. Some reptiles merely keep the eggs inside the body for varying periods up to and after hatching time. In others there are extensive, placentalike connections with the tissues of the maternal oviduct, and is used primarily for respiration. In a more advanced type the embryonic membranes, the chorion and allantois, interfold with maternal tissues and the embryo not only gets water and nourishment as well as oxygen, but conveniently has its excretory wastes taken away too. None of the live-bearing reptiles has dispensed with a big store of yolk as the main source of nourishment for the growing embryo.

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