Apparatus for Effecting Fertilization
Fertilization of Animals and Plants Contrasted
Fertilization, or the union of egg and sperm, initiates the life history though not the life of a new individual. Among plants the traveling male gamete, carried in the pollen cell on its way to the female gamete in the ovule, usually takes the air route, or steals a ride on some accommodating insect that acts the part of matchmaker.
Such methods are not employed in the fertilization of the eggs of animals. The sperm cell in the case of all vertebrates goes forward in its search for the ovum by means of a vibratile tail that requires a fluid medium in which to travel.
Naturally, the problem is least complicated in the case of those vertebrates that find themselves in water when the germ cells are expelled from the body, since a suitable medium for the locomotion of the sperm is ready at hand. When, in the course of evolution, emergence onto land took place, the reproductive apparatus had to be greatly supplemented in order to bring egg and sperm together for the purpose of fertilization. It is evident that the sperm of non-aquatic animals must encounter the eggs while they are still in the body of the female, since sperm cells cannot locomote through the air. This necessitates the pairing of the sexes by actual contact, so that the sperm may be definitely placed within the oviduct, rather than by the promiscuous broadcasting of germ cells in the surrounding medium followed by chance union, as in aquatic vertebrates. Moreover, a fluid, indispensable for the lashing tails of the locomotor sperm cells, must be elaborated from special glands provided for the purpose in order that the distance within the oviducts between the entering sperm and the descending ovum may be successfully covered.
Whenever internal fertilization occurs, the egg is thereafter either provided with a protective shell and laid, to be subsequently incubated and hatched (oviparous animals), or it undergoes its preliminary development within the sheltering walls of the female body before venturing alive into the world (viviparous animals).
To insure the placement of the sperm within the oviduct, various holdfast mechanisms, as well as organs of copulation, have been evolved in different animals.
The amphibians, which are halfway land animals, have been so occupied with adapting themselves to a dual existence that apparently they have not been able to accomplish much in an evolutionary way for their accessory reproductive apparatus. The result is that for the most part they return to the water during the breeding season, whore fluid for the locomotion of the sperm cells is freely provided, and there is no necessity for elaborating organs to accomplish internal fertilization.
Amphibians do, however, improve upon the improvident and wasteful ways of most fishes, with their countless eggs and unthinkable numbers of sperm. For example, frogs practice amplexation during the breeding season, that is the male frog saddles himself on to the back of the female, whose body, unfettered by hooplike ribs, becomes more and more swollen by the increasing mass of eggs within. The male retains his grasp upon the slippery back of the female by means of temporarily roughened glandular swollen thumbs, which are inserted under the armpits of the female and act as holdfasts. He remains in amplexation until the female extrudes the eggs, when he immediately sheds the sperm over them. In this way the hazards and uncertainties of distance between the germ cells of the two sexes are greatly lessened.
The hylas, or tree frogs, and the “obstetric frog” Alytes, of Europe, do not ordinarily resort to the water to breed, but Kammerer reports that when Alytes is compelled to breed in water, amplexation occurs and the thumbs of the male become swollen and roughened as in other frogs.
Many male lizards possess a row of peculiar porthole-like “glands” down the inner surface of the hind legs. These femoral structures are not true glands, producing a liquid secretion. Instead they extrude a dry scaly substance that roughens the surface of the legs so that when the male lizard in mating grasps the female between his hind legs, the security of his grip is insured.
Snakes coil around each other in the mating embrace. The male boa-constrictor, according to Boas, is supplied with anal hooks that aid in holding together the cloacas of the two sexes during the transfer of sperm to the female.
Among higher vertebrates, organs already present but not especially evolved to accomplish the holdfast function in reproduction are utilized for this purpose. Thus, the cock when treading a hen employs his beak and claws. In animals like ruminants, where holdfast organs would be difficult to imagine, the sexual act is usually accomplished very quickly, as in the case of the tumultuous and almost instantaneous leap of the stag. Camels solve their copulation difficulties by assuming a sitting-down posture.
Male Copulatory Organs
Copulation consists of the introduction of a male copulatory organ, the penis, in which the sperm duct terminates, into the enlarged end of the oviduct, or vagina. The discharge of the sperm cells under these circumstances makes more certain their placement in the immediate vicinity of the eggs to be fertilized.
It is to be expected that the apparatus for copulation in the aggressive male with sperm cells to deliver should be more elaborated than that of the receptive female, and such is the case.
Not all the organs for copulation among vertebrates are homologous. Thus, male elasmobranchs of many species possess a pair of “claspers,” one on each side of the cloaca, which are modifications of the medial parts of the pelvic fins (Fig. 396). In these animals, whose young are “born alive” in an advanced stage of development, copulation and internal fertilization are effected by the aid of these claspers, equipped with hooks, which are inserted into the cloaca of the female and serve to direct the sperm along their grooved inner margins into the oviducts (Fig. 397).
Rarely a similar contrivance is found among teleosts, that serves as an intromittent organ. This is formed sometimes out of the ventral or anal fin, as in the strange Brazilian bony fish, Girardinus, and sometimes from the metamorphosed haemal spine of a caudal vertebra.
It is with land life, however, that the evolution of internal fertilization and attendant copulatory mechanisms really begins, although none of the amphibians have a penis, with the possible exception of the footless caecilians. In these tropic amphibians, which have taken to land life in damp situations, the walls of the male cloaca during copulation are everted into the cloaca of the female, thus serving as an intromittent device.
Internal fertilization without the assistance of copulatory organs is accomplished among certain urodeles, Triturus, Amblystoma, Triton, and the viviparous Salamandra atra of Alpine streams, by a very different method. The males during the breeding season deposit their sperm in small compact packets or spermatophores, surrounded by a protective gelatinous mass, while the females follow after the males as the spermatophores are produced, and use their cloacal lips to pick up the spermatophores, even when the latter are deposited on land as sometimes occurs.
Among modern reptiles two types of penes have been evolved. First, among lizards and snakes, double cloacal organs are found, which may be everted somewhat after the manner employed by caecilians. Each hemipenis has a spiral groove along its medial surface for the conveyance of the sperm. Second, turtles (Fig. 398A) and crocodiles have a single penis, lying along the ventral wall of the cloaca, slightly protrusible and supplied with erectile tissue, dorsally grooved along its length. During copulation this groove is made into a temporary canal by its contact with the upper dorsal wall of the cloaca.
The single penis of the crocodiles is formed by two component parts suggesting a double origin. In turtles the size of the penis bears a distinct relation to the difficulties encountered in copulation because of the awkward shell. It is smaller in marine turtles having a flattened or incomplete plastron, than in fresh-water or land forms with a more complete and overarching shell.
Among birds there is no true copulation, but impregnation by the method of the cloacal kiss is the general rule. There are, however, a few birds, notably ostriches and other ratitates, certain ducks, and the South American “tinamou,” which have well-developed penes, resembling those of crocodiles.
The penis in all mammals, with the exception of monotremes (Fig. 398b), is a closed tube and not a grooved structure as in reptiles. Intermediate forms between reptiles and mammals are not known. It is typically a turgescent organ under the control of vasodilator nerves which arise as autonomic fibers in the sacral region of the spinal cord, and is composed of two masses of erectile tissue side by side, the corpora cavernosa, with a third similar mass, the corpus spongiosum beneath them, held together by fibrous tissue and enveloped by a layer of loose skin (Fig. 399). The corpus spongiosum is perforated for its entire length by the urethra. Consequently the urethra is considerably longer in the male than in the corresponding female. It terminates in an enlargement of the corpus spongiosum called the glans (Fig. 398d), which is split in marsupials where the female has a double vagina.
In some animals, for example the cat, guinea pig, and wombat, the glans is beset with horny recurrent spines or corneal scales that stimulate the female during copulation, although in most cases the skin extending over the surface of the glans is extremely sensitive and delicate without any hard corneal layer. Ruminants, cetaceans, and some rodents are exceptional in that there is no glans present, while sheep, goats and many antelopes possess a peculiar threadlike elongation of the sperm duct beyond the end of the penis itself, called the processus urethralis (Fig. 400).
Among higher animals the glans is enveloped in a double fold of retractile skin, the preputium (Figs. 384 and 398d). This is present only in forms with a pendent penis and is homologous with the sheath in which the penis is withdrawn in many animals such as ungulates.
In several kinds of mammals, namely, marsupials, cetaceans, moles, carnivores, rodents, bats, seals, lemurs, monkeys, and some apes, there is present in various degrees of development, lying in the connective tissue between the two corpora cavernosa and above the urethra, a penis bone, os priapi, which increases the rigidity of the organ which in most cases is attained by temporary turgidity caused by an influx of blood.
Among mammals the obliteration of the cloaca goes hand in hand with the evolution of the penis, or phallus (Fig. 398). In monotremes a cloaca is still present, and the small non-protrusible penis is enclosed in a sac between the urogenital sinus and the cloaca itself. It is a fibrous, slightly erectile structure of double origin, with a groove between the two parts that is converted into a canal except posteriorly where the urogenital and cloacal passages still connect with each other. The ureters, instead of terminating in the bladder with a urethral outlet for the urine, empty through a urogenital sinus into the cloaca, the penis being utilized solely for the conduction of the sperm.
The marsupial penis, with the reduction of the cloaca, becomes an external erectile structure directed backward, through which both urine and sperm have common passage-way, as in all higher mammals. The scrotum containing the testes is located anterior to the penis. The distance between the penis and the anus is so slight in these animals that both penis and anus are enclosed together within a common muscular sphincter. The opossum Didelphys, the bandicoot Perameles, and some other species, have a bifurcate glans that is probably correlated with the double vagina in the females of these forms.
Among placental mammals the penis is in front of the scrotal sac and usually withdrawn within a protective sheath, except during sexual activity. In most cases it is directed forward and, when no sheath is present, is pendulous, as in bats and man. In monkeys and apes it is partly pendulous and partly ensheathed. In cats and rodents it is directed backward, except during copulation, so that these animals micturate posteriorly. Armadillos, handicapped by their awkward armor, have a relatively enormous penis which may extend as much as one third of the length of the body during copulation.
The female genitalia concerned in copulation, aside from certain glands and the rudimentary clitoris, are the vagina, vestibule, hymen, and the labia minora and majora (Fig. 401). All of these structures are differentiated to a point comparable with the degree of evolution attained by the copulatorv organs of the corresponding males, reaching their maximum in the primates.
The vagina is that part of the oviduct adapted to receive the penis. It is absent in the monotremes where the cloaca serves the same purpose. In the opossum, Didelphys, and other marsupials, there are two distinct vaginas which barely coalesce into one at the outer entrance, while in some other marsupials there is a coalescence at the inner ends from which a median diverticulum, or third vagina, extends posteriorly as a blind alley between the two lateral vaginas (Fig. 402). Whenever a fetus becomes deposited in this closed middle vagina instead of in one of the two lateral open passage-ways, there must be a rupture at birth at the blind end of the middle vagina to allow for the expulsion of the young.
In placental mammals the outer ends of the two oviducts open into a single vagina, located between the rectum dorsally and the urethra ventrally. It is lined with mucous membrane, frequently crossed by transverse rugae, particularly in young individuals. The vaginal mucosa is without glands, the mucus that is present there coming from the walls of the uterus, especially the cervix, which is continuous with the vagina. The walls of the vagina are muscular and collapsible, and the muscular fibers near the external orifice form a sphincter.
The outer part of the vagina constitutes the vestibule, which is separated from the vagina proper by a temporary fold of mucous membrane, more or less complete, called the hymen. This is the real end of the embryonic Mullerian duct, and it partially occludes the passage-way, particularly before copulation has taken place. Originally in all mammals, the hymen persists in bears, seals, hyaenas, and most apes, but is present only in the young of the horse, pig and mole.
The walls of the vestibule are supplied with erectile tissue which at times becomes surcharged with blood. In primates generally around the vestibule on either side are two folds of skin, the labia minora, while in the higher primates there are in addition two external folds, the labia majora, covered outwardly with pubic hair and supplied with a certain amount of fatty tissue. There is also a pad of adipose tissue anterior to the vestibular orifice at the edge of the pubic bones. Likewise covered with hair, it is called the mons Veneris. It is better developed in the human females of civilized races than in those of more primitive types. The female bushman is remarkable for the hypertrophy of the labia minora, as well as for the enlarged posterior gluteal muscles, resulting in enormously prominent buttocks, or the condition described as steatopygy (Fig. 403).
The clitoris in the female is homologous with the penis in the male. It is situated just anterior to the vestibular opening between the folds of the labia minora and consists of two corpora cavernosa that are erectile, but there is no corpus spongiosum present, and the clitoris is not traversed by the urethra like the male penis, except in a few rodents, insectivores, and lemurs. In this latter case the urethra is not a urogenital canal, as in the penis, but is exclusively for the transmission of urine, as in other mammals. An imperforate clitoris is present in turtles, crocodiles, and a few exceptional birds whose males possess a penis. There is even a clitoris bone in certain mammalian species that corresponds to the os priapi of the male. The clitoris, which is relatively large in monkeys, being six or seven centimeters long in the young spider monkey, Ateles, is comparatively small and degenerate in the human female.