Fig. 229.—Diagrammatic transverse sections through an embryo Craniate to show the mode of development of the pronephros (A) and of the mesonephros (B). The right side of each figure shows an earlier stage than the left. In B (left side) the connexion of a vas efferens with a mesonephric tubule, and the division of the archinephric duct into Müllerian and mesonephric ducts are shown, a, Aorta; a.c, alimentary canal; a.d, archinephric duct; g, glomus; gl, glomerulus; i.n, inner nephrostome; mb, Malpighian body; md, Müllerian duct; mnd, mesonephric duct; mnt, mesonephric tubule; myc, myocoele; myt, myotome; n, notochord; np, nephrotome; nt, nephrostome; o.n, outer nephrostome; pn.t, pronephric tubule; s.c, spinal cord; t, testes; v.c, ventral coelom; v.ef, vas efferens. (After Kingsley and Semon.)
At the same time the nephrotomes lose their connexion with the myocoeles, although they still retain their "nephrostomes" or apertures through which they communicate with the ventral coelom. When fully developed the pronephros consists of a few tubules, more or less convoluted, opening at their inner extremities into the coelom by means of their ciliated nephrostomes, and at their outer ends communicating with the exterior through the archinephric duct. In relation with the pronephros a branch from the dorsal aorta forms a tuft of capillary blood-vessels or "glomus," opposite the nephrostomes, which projects into the ventral coelom on each side. Later, a second series of much more numerous tubules is formed behind the pronephros, which constitute the mesonephros. In forming mesonephric tubules the nephrotomes become disconnected from the myotomes and their myocoeles, and curving outwards they come to open into the archinephric duct, although they do not in any way contribute to its formation (Fig. 229, B). Segmentally-arranged twigs from the dorsal aorta end in tufts of capillaries or glomeruli, each of which projects into a small sac-like enlargement of a mesonephric tubule, pushing before it the wall of the sac. In this way a double-walled "Malpighian body," containing a "glomerulus," is formed in connexion with each tubule. Subsequently, the mesonephric tubules increase in number by budding. New nephrostomes and Malpighian bodies are developed on the secondary branches, and the original segmental arrangement of the tubules becomes obscured. With the growth of new tubules, and the formation of blood-vessels and of connective and lymphoid tissues between them, each mesonephros finally assumes the condition of a compact gland imbedded in the dorsal wall of the coelom, with its ventral surface invested by the peritoneum. A "metanephros," which in the higher Vertebrates replaces the mesonephros as the functional kidney, is perhaps not represented in Fishes.
A more or less well-developed pronephros is present in the embryos or larvae of the Cyclostomes and of all Fishes, but as a rule it completely disappears at an early period and is replaced by the mesonephros. It is retained throughout life, however, in the Myxinoid Cyclostomes (Fig. 230, B), and has its persistent nephrostomes opening into the pericardial cavity.[[475]] In a few Teleosts the pronephros is also persistent, as in Fierasfer and Dactylopterus, and in others the organ may not completely disappear until the approach of sexual maturity. But with these exceptions the mesonephros is the sole functional kidney in the adults of the Cyclostomes and of all Fishes. As regards the nature of the duct by which the excretion of the mesonephros is conveyed outwards, there are notable differences in different Craniates. The Cyclostomes and the Teleostomi retain that part of the archinephric duct into which the mesonephric tubules open, and which remains after the atrophy of the pronephros (Fig. 230, B, E, F). In Elasmobranchs, and probably also in the Dipnoi, a special mesonephric duct is developed in a way which will be described later (Fig. 230, C, D).
Fig. 230.—Showing the principal modifications of the kidneys and reproductive organs in Cyclostomes and Fishes. A, The pronephros and its duct in the embryo; B, the kidneys and genital pores in Petromyzon, the vestigial pronephros represented as in Myxine; C and D, the urinogenital organs of a male and female Elasmobranch; E, of a male or female Teleost, or a male Lepidosteus; F, of a female Polypterus, Acipenser, Amia, or Osmerus. a, Anus; a.d, archinephric duct; c, cloaca; c.a, the coelomic aperture of the Müllerian duct; c.p, cutaneous pit; g, gonad; gd, gonoduct; g.p, genital pore; i, intestine; m, Malpighian body; m.d, Müllerian duct; mn, mesonephros; mn1, vestigial mesonephros; mn2, excretory portion of the mesonephros ("metanephros"); mn3, genital portion of mesonephros; mn.d, mesonephric duct; mtn.d, metanephric duct; n, nephrostome; ov, ovary; p.a, abdominal pore; p.f, peritoneal funnel; pn, pronephros; pn′, vestigial pronephros; s.g, shell gland; s.s, sperm sac; t, testis; ug.s, u.s, urinogenital sinus; v.ef, vasa efferentia; v.s, vesicula seminalis.
In the males of Elasmobranchs some of the hinder mesonephric tubules unite to form a single main duct opening into the terminal part of the mesonephric duct, and these tubules and their separate duct are sometimes regarded as a metanephros and a metanephric duct. The mesonephric nephrostomes are persistent throughout life in a few Elasmobranchs (e.g. Notidanidae, Heterodontidae, Rhinidae, and some Scylliidae), and also in Amia:[[476]] in all other Fishes as well as in the Cyclostomes they become closed in early life.
Fig. 231.—Diagrammatic horizontal section through the abdominal pores and cloaca of an Elasmobranch. a.p, Abdominal pore; c, coelom; cl, cloaca; cl.p, cloacal papilla; c.p, cloacal pit; od, oviducal apertures in the female; r, rectum; u.s, cloacal aperture of the urinary sinus (female), or the urogenital sinus (male). In some Elasmobranchs the abdominal pore opens at the base of the cloacal papilla, as shown at a.p1. (Modified from Bles.)
In many Fishes the hinder extremity of the coelom communicates directly with the exterior through "abdominal pores," of which there is usually a pair, rarely a single pore, situated close to the cloacal or the anal aperture.[[477]] Elasmobranchs usually have a pair, often at the extremities of a pair of cloacal papillae (Fig. 231), but they are absent in some families (e.g. Heterodontidae and Rhinidae); and in some Scylliidae (e.g. Scyllium canicula) they are very variable, being either present or absent on both sides, or an open pore is present on one side only. Pores are present and paired in the Crossopterygii, the Chondrostei, and the Holostei. Amongst the Dipnoi Neoceratodus has a pair of pores. Protopterus sometimes has two pores opening into the cloaca, but as a rule the two become confluent and have a single external aperture. In Lepidosiren pores are wanting. Abdominal pores are rarely present in Teleostei. They exist, however, in the Mormyridae (Gymnarchus and several species of Mormyrus), and also in the Salmonidae,[[478]] where they are as singularly variable in different species and individuals as in the Elasmobranch Scylliidae. The use of abdominal pores is not certainly known, unless the coelom of those Fishes which possess them continues to retain some measure of its primitive excretory function, and the pores act as excretory ducts. That the nephrostomes are excretory organs has been shown by experiment, and it is worthy of note that there exists a reciprocal relation between these structures and abdominal pores, to the extent that while there are a few Fishes (e.g. certain Elasmobranchs and Amia) in which both coexist, there are many others in which the presence of nephrostomes is correlated with the absence of pores and vice versâ.