The genital glands are situated anterior to the kidneys, the right extending farther forward and often larger than the left. The testes are elongate. The vas deferens is closely folded proximally, and runs along the outer side of the kidney into the cloaca close to the ureter. The ovaries are elongate, and consist of two lamellÆ, with a lymph-space between them. The oviduct extends from near the anterior extremity of the ovary to a common chamber, or vagina, which is above the rectum and opens into the cloaca; this vaginal chamber may be more or less completely divided into two.

The males are provided with a pair of intromittent organs, or hemipenes, each connected with one of the caudal vertebrÆ by a muscle (retractor penis) which often exceeds it in length. These organs are cylindrical or club-shaped and hollow, with the inner surface divided into numerous cavities and beset with papillÆ, and usually also with hard spines, of which those towards the apex may be greatly developed, folded against the walls, and directed towards the extremity. Such spines are absent in the snakes provided with claw-like rudiments of hind limbs. The cavities of the hemipenis are connected by a branch with the dorsal artery, and it is by a flow of blood into them that erection of the organ is accomplished. Each hemipenis is lodged in a cavity on each side of the base of the tail; when protruded it turns inside out, and the inner surface becomes the outer, the papillÆ and erected spines serving to maintain a firm hold in the vagina, from which the organ cannot be withdrawn except by invagination. It has been observed that the presence of spines on the hemipenis is associated with much tougher vaginal walls. The organ is grooved along its entire length, the groove being the sulcus spermaticus, which, when the edges of the two hemipenes meet, forms with its fellow a canal to convey the semen into the oviduct; this sulcus may be bifurcate, as in the Viperids and some Colubrids.

Anal pockets, secretory organs on each side of the vent and lodged in the base of the tail, seem, in females, to be the homologues of the hemipenes; but this view cannot be held, since the same organs are present, though smaller, in males also, situated dorsally to the hemipenes. The glands with which they are provided produce the strong and offensive odour which appears to be a means of defence in our Grass-snake and other species, and which also serves to bring the sexes together, the glands being more active during the breeding season. A Viper-catcher in France is said to obtain good results by rubbing his boots with these glands, as a means of attracting the snakes in the spring.

In European species pairing takes place in spring, sometimes again at the end of summer or in autumn. After hibernation the testes of the males are rather voluminous, and the sperm-ducts are often full of spermatozoa. The male gets alongside the female, sometimes seizing her round the neck with his jaws, and remains stretched out against her or twists the posterior part of his body in a few coils around hers. In the Vipers the bodies of the pairing individuals are completely entwined. The male then endeavours to bring the two anal orifices together, and when he has succeeded in getting the female to distend her cloacal opening, the intromittent organs are suddenly everted into the vagina. The union of the sexes sometimes lasts only a few minutes, but usually an hour or more; it has even been observed to last a whole day. Several copulations may take place at intervals of a few days. Many snakes are gregarious during the breeding season, and great numbers of males have been seen wriggling round the females, forming with their coils huge lumps or an entangled mass like a ball. The more or less prehensile tail with which thoroughly aquatic snakes, such as Hydrophis and Acrochordus, are provided, is no doubt of use in facilitating the pairing, when it has to take place in the water. Our European Water-snakes pair on land.

During the rutting season a slight pressure on the base of the male’s tail may cause the protrusion of the hemipenes, and so may a violent blow on the spine of the reptile. Thus, recently killed specimens of our Adder, with the organs everted, have more than once been taken by the ignorant for snakes with hind limbs, a mistake which must be pardoned when we remember that male embryos of the slow-worm and of snakes, in which the hemipenes are normally everted, have been described by zoologists, who should have known better, as examples showing external vestiges of limbs.

The spermatozoa soon make their way up the oviducts, in which the ripe ova have previously descended, or which gradually descend shortly after, these ducts becoming dilated in consequence. There are usually more eggs in the right than in the left oviduct, although the reverse has occasionally been observed.

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.

The number of eggs or young of one brood varies much according to the species, and also according to the age of the mother, large females usually producing a higher number and of a larger size than smaller specimens of the same species. Our European Zamenis, Coluber, and Coronella produce only 2 to 15; our Tropidonotus, 15 to 48; our Vipers, 3 to 22. Among exotics we may mention, as the most prolific, Bitis nasicornis, up to 47 young; Tropidonotus fasciatus, Abastor erythrogrammus, and Farancia abacura, 50; Lachesis lanceolatus, 60; Vipera russelli, 63; Boa constrictor, 64; Tropidonotus ordinatus, 78; Pseudaspis cana, 80; Python molurus, nearly 100 eggs.

The eggs are deposited in holes without any sort of nest, under moss or decomposing leaves, in accumulations of saw-dust, or in manure-heaps. In many cases it has been observed that the female remains for some time with her eggs or young, and in the large Pythons a sort of incubation takes place, the female remaining coiled in a spiral over the mass of eggs for six to eight weeks; an increase of several degrees in her temperature at that period has been ascertained by experiments conducted with every possible care, a remarkable fact in the case of a so-called “cold-blooded” animal.

The numerous reports of young snakes seeking refuge in their mother’s gullet have not been substantiated by satisfactory scientific evidence, and, although it is perhaps wise to say that the question remains an open one, it may be mentioned that, in Europe at least, trained observers who have devoted special attention to the habits of Vipers, in districts where these reptiles are exceedingly abundant, have never come across an instance of the form of maternal solicitude with which these snakes in particular have been credited. Not a single reported case of a female snake swallowing her young for protection rests on satisfactory evidence.

The embryo is closely coiled up in a spiral. Just before birth it is distinguished by a large, convex head, with large, prominent eyes, and a comparatively short body, the scales and ventral shields being much shorter than later in life. The umbilicus is situated in the posterior part of the body, from six to ten times as far from the head as from the vent. Long after birth the umbilical slit remains visible, and affords a means of distinguishing very young snakes from older examples of smaller species. In oviparous species the embryo is provided with a very conspicuous egg-tooth, pointing forwards and projecting from the notch in the lower border of the rostral shield; this egg-tooth is much reduced, and sometimes very indistinct, in the ovoviviparous species. The function of the egg-tooth is to cut through the tough eggshell. This, after the young has left it, shows one or several slits in its anterior extremity, cut as clean as if with a sharp knife. The egg-tooth becomes loose soon after birth, and is shed within a few hours or a few days, sometimes even before birth in ovoviviparous species.

Frequent cases have been observed of dicephalous embryos or young, which may live for a short time; there are even records of a three-headed snake, stated to have been seen at Lake Ontario, and of snakes with two heads and two tails.

Unless prematurely born with a considerable mass of vitellus attached to the umbilicus, the young immediately after birth resent all interference, hissing, snapping, or puffing themselves up, after the manner of their parents. The first shedding of the outer coating of the epidermis follows soon after birth; not before then does the young take to food.

No snake appears to be able to breed before it is four years old.

Well-authenticated instances of different species interbreeding are unknown, but specimens intermediate between Vipera berus and V. aspis, and between V. berus and V. ammodytes, have been assumed, with much probability, to be hybrids.