The male and female gonads, testes and ovaries, are derived from the coelomic epithelium near the inner or median aspect of the nephrotomes (Fig. 229, B). Here the epithelium remains columnar, and soon projects into the ventral coelom as a continuous longitudinal ridge. It is probable that at first the modified epithelium is segmented as a series of "gonotomes," but if so, the latter must soon coalesce into a continuous ridge. Some of the epithelial cells enlarge to form the primitive sex-cells. In the development of an ovary, portions of the epithelium sink inwards, carrying with them the primitive ova. Certain of the cells form the epithelial walls of a number of ovisacs, each of which encloses an ovum. As the ovisacs increase in number and size the germinal ridges project more and more into the coelom until, as ripe ovaries, they become suspended from its dorsal wall by a double peritoneal fold, the "mesovarium" (Fig. 156). The testes develop in a similar fashion except that the primitive sex-cells, which later give rise to spermatozoa, form the lining of a number of simple or ampulla-like tubules, the seminiferous tubules, and the suspensory fold is termed the "mesorchium."
The Cyclostomes have gonads in the shape of unpaired organs extending nearly the whole length of the coelom, but in all Fishes the organs are primarily paired, although by fusion, or by the absorption of one gonad, the ovaries or the testes sometimes appear as if single. The ovaries may either be naked, as in Elasmobranchs, Dipnoi, Crossopterygii, and Chondrostei, and in Amia amongst the Holostei; or, as in Lepidosteus and most Teleosts, they become enclosed in coelomic sacs. The former, or "gymnoarian," condition is primitive; the latter, or "cystoarian," is secondary, and is brought about by the growth of two peritoneal folds round the ovary and the union of their margins. Into these coelomic sacs the egg-bearing or real ovarian tissue projects either in the form of processes or of transversely- or longitudinally-arranged plates or folds (Fig. 232, B). The testes are composed of seminal ampullae, as in Elasmobranchs, or of radially-arranged and sometimes plexiform tubules opening into the gonoduct, as in nearly all other Fishes (Fig. 232, A).
Fig. 232.—Diagrams to show the structure of the testes (A) and of the ovaries (B) in a Herring. (From Cunningham.)
In the Cyclostomes (e.g. Petromyzon) the eggs and spermatozoa are discharged from the gonads into the coelom, whence they reach the exterior through a pair of "genital pores" leading from the hinder end of the coelom into a urinogenital sinus formed by the united extremities of the two archinephric ducts.[[479]] Myxine has, however, but a single median pore, opening into an integumentary cloaca, which also receives the rectal and urinary orifices. Bdellostoma has two such pores communicating with a similar cloaca.[[480]]
The nature and homologies of the genital ducts in the different groups of Fishes are amongst the most puzzling of the many problems which vex the soul of the Vertebrate morphologist, and although there is a fairly general agreement on some points, there are others of great importance of which it may be said quot homines, tot sententiae.
Broadly speaking, there are two types of genital ducts in Fishes: (1) those which are obviously derived from some part of the kidney system; and (2) those which are special ducts and appear to have no connexion with kidney-ducts.
The Elasmobranchs offer a typical example of gonoducts of the first kind. At an early embryonic period in both sexes each archinephric duct becomes longitudinally split into two ducts, of which one continues to receive the openings of the mesonephric tubules and remains as a mesonephric duct (Fig. 229, B).[[481]] The other, which has no connexion with the mesonephros, opens anteriorly into the coelom by means of the united nephrostomes of the pronephros, and is known as the "Müllerian duct" (Fig. 230, C and D). In the adult male the Müllerian ducts are useless vestiges, but in the female they persist and act as oviducts, receiving the eggs set free from the ovarian ovisacs through their coelomic apertures, and thence conveying them to the cloaca. In the male, certain of the anterior mesonephric tubules become connected with the testicular ampullae by means of a network of slender tubules, the "vasa efferentia" or testicular network, and through the latter the spermatozoa pass from the testes to the mesonephric duct (Fig. 230, C). Consequently, the mesonephric duct conveys both spermatozoa and the kidney excretion to the cloaca. It is obvious, therefore, that both the male and female gonoducts are derived from kidney-ducts.
The Teleostei afford an equally typical illustration of the second type. Each female gonoduct (oviduct) is formed by a backward growth of the same two peritoneal folds which enclose the ovary; these are converted into a "peritoneal tube" or canal by the union of their margins. The male gonoducts are also formed in continuity with the testes, that is, as backward prolongations from the latter. Each duct, male or female, seems to be a duct sui generis and to have no connexion whatever with the kidney system (Fig. 230, E). In the Salmonidae, Anguillidae, Galaxiidae, Hyodontidae, Notopteridae, and Osteoglossidae, and also in Misgurnus, the oviducts lose their continuity with ovaries and degenerate to an extent which differs greatly in different families. Thus in some Salmonidae, as in the Smelt (Osmerus eperlanus),[[482]] the oviducts end anteriorly in wide funnel-like coelomic apertures after the fashion of Müllerian ducts, and do not embrace the ovaries: hence the ovaries are naked and not cystoarian, and their ducts are not peritoneal tubes but "peritoneal funnels" (Fig. 230, F). In other Salmonidae and in the Anguillidae the oviducts appear to have so far degenerated that they are represented either by a pair of very short funnels or by a pair of genital pores, which, as in the Salmon, have a common external aperture behind the anus and in front of the single orifice of the united archinephric ducts (Fig. 233, A). In all such instances the eggs are set free from the ovaries into the coelom, from whence they escape through the peritoneal funnels or genital pores. In the Eels the male gonoducts also degenerate, and, losing all connexion with the testes, they become reduced to genital pores as in the female.
The Holocephali and probably the Dipnoi conform to the Elasmobranch type in the nature of their male and female gonoducts. In the Crossopterygii[[483]] each testis has its own proper duct, which has no connexion with the kidney system and apparently belongs to the Teleostean type, while the oviduct, which is almost certainly not a Müllerian duct, is probably a peritoneal funnel. On the other hand, the Chondrostei and the Holostei are in the interesting transitional condition of possessing male ducts of the Elasmobranch type and female ducts of the Teleostean type, the latter being either ducts directly continuous with the ovaries, as in Lepidosteus, or of the nature of peritoneal funnels, as in Acipenser, Polyodon, and Amia (Fig. 230, E and F).