All the live-bearing reptiles of modern times are lizards and snakes. Turtles and crocodilians produce only eggs, and so does the tuatara. It is significant that 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 slowworm (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. In one type the yolk sac is merely plastered against the wall of the 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 nourishment for the growing embryo.
All reptiles practice internal fertilization. In all modern forms except the tuatara the male has an organ kept turned outside in, in the base of the tail, and everted through the opening of the cloaca during erection. In the tuatara the transfer of sperm is accomplished by bringing the genital openings into contact, as in birds. This was probably the method used by the ancestral reptiles – it is clear, in any case, that the penis had separate origin in turtles, crocodilians and mammals on the one hand, and in lizards and snakes on the other.
Thus, male lizards and snakes have not just one, but a pair of hollow structures called hemipenes, which make up their copulatory organs. located as they are in the tail just behind the opening of the cloaca, the hemipenes often give the tail of the male a thicker, more gradually tapering contour than that of the female, and in many species the sexes can be distinguished by this difference. A groove that serves as a channel for the sperm extends from the opening of the sperm ducts along the inner wall (which is the outer wall during erection) of each hemipenis, and the surface may be pleated or set with spines that keep it in place in the oviduct of the female during mating. Either one of the hemipenis may be used, but only one, the one nearest to the female, is everted and protruded from the cloaca during erection, which is brought about by a combination of muscular action and distension of the walls with blood.
Among different reptiles fertilization is scheduled differently with respect to the time of nesting. In most species it seems to occur, as might be expected, just before the eggs are laid; but in some the sperm may live on in the reproductive tract of the female and continue to fertilize eggs months or even years after copulation has taken place. The longest known periods of such deferment of fertilization are four years for the diamond back terrapin of the southern United States, and five years in the case of the tropical American cat-eye snake. The green turtle, which evidently mates only in the sea off the nesting beach, often does so after the female has gone ashore and laid her eggs. Since a given female makes her migration to the nesting ground only once in three, or more turtles.
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