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withdrawn except by invagination.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.
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.
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