Disposal of Gametes
The body cavity may be regarded as an expanded gonadal sac, since the sex cells come to maturity on its inner wall and are shed into it. There are at least three general ways among vertebrates by which the mature gametes finally escape to the outside, namely, (1) through pores in the body wall, no ducts being present; (2) through ducts continuous with the gonads; and (3) through ducts not continuous with the gonads and for the most part taken over from the mesonephridial apparatus.
The first of these avenues of escape is seen in amphioxus, whose gametes are extruded into the peribranchial chamber and thence out through the atrial pore. In some cyclostomes also, which receive eggs and sperm into the body cavity, abdominal pores furnish an exit through the posterior part of the body wall into the urogenital sinus behind the anus (Fig. 391). These pores are usually paired but may, as in Petromyzon, present a single opening through a projecting papilla. The method of disposal of sex cells through abdominal pores is also utilized by a few teleost fishes belonging to the Anguilliformes, the various types of eels. Here, however, it is evidently a degenerate condition.
Many teleost fishes employ the second method of germ-cell disposal, that is, by means of direct ducts that are continuous with the gonads. The ova never enter the main body cavity but pass out directly through ducts united with ovarian sacs which entirely invest the gonads. It will be recalled that the cavities of these sacs are pinched-off portions of the original main body cavity. Sperm cells are carried from the testis to the urogenital sinus by a genital duct which grows back from the testis completely independent of the mesonephros (Fig. 392).
The third method, that of indirect ducts, is by far the most common. In elasmobranchs, dipnoans, amphibians, reptiles, birds, and monotremes the ducts open into a cloaca, but in mammals other than monotremes, directly to the outside as a result of the longitudinal splitting of the cloacal region into a dorsal digestive portion and a ventral urogenital passage-way. Generally in this third group the sperm, never entering the body cavity at all, reach the sperm duct indirectly after passing through the long convoluted tubules of the epididymis, while the eggs always enter the body cavity from the ovary and are then picked up by the open ends of the unconnected oviducts for transference to the outside. There is, therefore, a fundamental difference in the disposal of the germ cells of the two sexes.
Exclusive of the teleosts there is an intimate relation, particularly in the male, between the ducts that dispose of the sexual products and the mesonephridial system. It will be remembered that the segmental ducts of the pronephroi (Fig. 391) remain after the pronephridia have been replaced by mesonephridia, to become the mesonephric, or Wolffian ducts (Fig. 393). These ducts not only drain the mesonephros, but in the case of male elasmobranchs and amphibians they also serve as sperm ducts, for which reason they are properly designated as urogenital ducts.
The connection of the urogenital duct with the testis is accomplished through the mesonephridia of the anterior part of the mesonephros, known as the “sexual kidney,” or epididymis, as distinguished from the posterior part, or “urinary kidney.”
In male reptiles, birds, and mammals only the sexual kidney remains functional as the epididymis, while the urinary part of the mesonephros, being replaced by the permanent metanephric kidney, becomes degenerate (Fig. 394). The Wolffian ducts, however, persist as sperm ducts exclusively, in the guise of the ducti deferentes, or vasa deferentia, while the urinary products of the metanephros are disposed of through independently formed ureters.
After leaving the walls of the tubuli eontorti in which they are originally embedded, the sperm cells pass through the tubuli recti, the rete testis, and ductuli efferentes to emerge into the epididymis. This latter is essentially an organ made up of several epididymal tubules, or cranial mesonephridia, and a very much coiled part of the Wolffian duct, the ductus epididymis. In reptiles the epididymidis is sometimes even larger than the testis itself.
The tubules in the epididymis are transformed mesonephridia which have lost their glomeruli and acquired a secondary connection with the rete testis through the so-called junctional cords of cells that later become hollowed out as the ductuli efferentes, making a continuous passage-way from the tubuli eontorti in the testis to the outside.
In man the epididymal tubules, of which there are from nine to fifteen in each epididymis, are very much coiled and together form the globus major, or caput, of the epididymis. They still empty into the end of the Wolffian duct as they did before being converted from mesonephridia and transferred to the service of the reproductive system. The beginning of the Wolffian duct is also greatly convoluted and constitutes a mass known as the globus minor, or cauda, of the epididymis. From the globus minor the Wolffian duct emerges as the ductus deferens. The entire Wolffian duct in man, according to Lewis, is about twenty feet in length if straightened out. When the infinitesimal size of the human sperm cell is taken into consideration, its journey to the outside world from its point of origin in the testis is a comparatively extensive one.
Near the larger open end of the Wolffian duct there is usually a saccular expansion called the seminal vesicle, which, although glandular in character, frequently serves the purpose of a temporary storage place for the traveling sperm, as its name would indicate.
In mammals there is a union of the urethra and the ducti deferentes beyond the points where the seminal vesicles open into them, so that the end of the sperm duct in an amniote, like the entire duct in the males of elasmobranchs and some amphibians, is urogenital in character, serving the double function of disposing of both sperm and urine.
That part of the ductus deferens between the seminal vesicle and the point of its junction with the urethra is termed the ejaculatory duct. Its muscular walls by peristaltic action evacuate the sperm.
The oviducts are less directly related to the mesonephridial apparatus than are the sperm ducts. Early in embryonic life a second duct, the Mullerian duct, appears alongside the Wolffian duct (Fig. 393). This becomes the oviduct. It may form by longitudinal splitting of the segmental duct, as in elasmobranchs, or it may develop parallel to the segmental duct out of surrounding tissues either as a groove in the peritoneum that becomes closed off, or as a solid strand of cells that afterwards becomes hollowed out. Thus, the segmental duct may either split to form the Mullerian duct and the Wolffian duct, or persist undivided as the Wolffian duct.
Except in teleosts each oviduct opens freely into the main body cavity by an ostium abdominale (Figs. 393, 394, and 396). In some cases these open ends of the ducts may be located at a considerable distance from the ovaries themselves and the points where the eggs dehisce, or they may be close around the ovaries as in mankind, so that, while the eggs theoretically pass through the body cavity, there is little chance that they may escape therein and miss the openings into the ducts. Once within the oviducts the eggs are forwarded by peristalsis of the muscular walls.
The ostium abdominale in elasmobranchs is probably derived from a persistent mesonephridium that attained embryonic connection with the segmental duct. Thus, the ostium abdominale, into which the ova from the body cavity are transferred, is homologous with the nephrostome of a nephridium that ancestrally served to drain the body cavity of fluids of excretion. There is no connection in other vertebrates, either in the male or in the female, between the embryonic Mullerian duct and the urinary apparatus.
In the exceptional teleosts there is an organic unity between ovary and oviduct, entirely preventing an excursion of the egg into the body cavity. In some carnivores, such as the walrus and seal, the ovary becomes so encapsuled within the lips of the ostium abdominale, that it superficially resembles the condition in teleosts.
Some elasmobranchs and marsupials have the open inner ends of the two oviducts fused together, forming a single ostium. Fusion at the cloacal ends of the two oviducts, however, is more common. It usually occurs in teleosts and certain Salientia as well as quite generally among the mammals.
In those animals that produce many eggs at one time, as the frog for example, the oviducts temporarily become enormously elongated and convoluted during the egg-laying season (Fig. 395). The oviduct of a pullet which has not begun to lay eggs is about six or seven inches in length, whereas in an egg-laying hen it is four times as long.
Among higher vertebrates the oviducts are made up of Fallopian tubes for the reception of the eggs from the ovaries, and nearer their exit they undergo extensive elaboration into uterus and vagina, modifications to be considered later.
Mullerian ducts are also present embryonically in the male, but in the adult they suffer various degrees of degeneration. The lungfish, Neoceratodus, and the newt, Triton, as well as the leopard frog, Rana pipiens, are exceptional cases, showing persistent Mullerian ducts in the adult males.