Fat-bodies are much developed, either in the form of small separate lobes, or as a continuous, much folded band, on each side of the body.
CHAPTER IX
ORGANS OF REPRODUCTION; PAIRING; OVIPOSITION; DEVELOPMENT
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the outer, the papillæ and erected spines serving to maintain a firm hold in the vagina, from which the organ cannot be withdrawn except by invagination.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.