Some snakes lay eggs shortly after impregnation, or a few weeks later; in others the young undergo their development within the oviducts, each enveloped in a thin, transparent, membranous capsule, which is torn immediately before or immediately after parturition, such species being termed "ovoviviparous." Just before oviposition the female curves the base of the tail upwards, in order to extend the cloacal opening. The eggs are all produced together, usually at intervals of a few minutes, and generally adhere to one another by means of a sticky fluid secreted by the oviducts, thus forming a clump. In ovoviviparous snakes the young are born in succession, in the course of a few hours or of a few days. In many oviparous species it is the rule for freshly-laid eggs to contain more or less developed embryos, and _Coronella punctata_ is said to produce thin-shelled eggs which hatch in less than half the time required for the eggs of its American congeners under the most favourable circumstances. There is thus almost every degree between oviparity and ovoviviparity. These two modes of parturition bear no relation to the natural affinities of snakes. Thus, the European _Coronella austriaca_ is ovoviviparous, and its North American congeners are oviparous; whilst, curiously, it is the inverse in the genus _Tropidonotus_. It was long believed to be an invariable rule for the Viperidæ to bring forth live young, the name Viper being derived from this well-known peculiarity, but it has now been ascertained that the South American _Lachesis mutus_, the Indo-Malay _Lachesis monticola_, and the African _Causus_ and _Atractaspis_, lay eggs. All exclusively aquatic snakes, such as the Hydrophiinæ, are ovoviviparous, and thus dispensed from going on land for parturition. The yolk entirely fills the eggshell; there is no albumen, or, if any exists, it is so much reduced as to easily escape observation. The eggshell in oviparous species contains a small amount of lime, and is not hard, but tough and parchment-like, white or yellowish; it is usually smooth, but in Pythons its surface is studded with minute pores, and in the American _Zamenis constrictor_ it is rough, as if sprinkled over with loose grains of salt. The shape varies from a short oval to a long ellipse. It has been observed in some snakes that the eggs, on leaving the cloaca, are of an elongate shape, suggestive of a short cigar, and immediately after assume a more oval form. After they have been laid, the eggs absorb moisture and thus increase in size, especially in width; eggs which are at first twice as long as broad may be almost globular just before the birth of the young.