Fig. 394. Portions of the mesonephros of Myxine. (From Gegenbaur; after J. Müller.)
a. segmental duct; b. segmental tube; c. glomerulus; d. afferent, e. efferent artery.
B represents a portion of A highly magnified.
The portion of the body cavity with the glomerulus and peritoneal funnel of the pronephros ([fig. 395], po) soon becomes completely isolated from the remainder, so as to form a closed cavity (gl). The development of the mesonephros does not take place till long after that of the pronephros. The segmental tubes which form it are stated by Fürbringer to arise from solid ingrowths of peritoneal epithelium, developed successively from before backwards, but Sedgwick informs me that they arise as differentiations of the mesoblastic cells near the peritoneal epithelium. They soon become hollow, and unite with the segmental duct. Malpighian bodies are developed on their median portions. They grow very greatly in length, and become much convoluted, but the details of this process have not been followed out.
The foremost segmental tubes are situated close behind the pronephros, while the hindermost are in many cases developed in the postanal continuations of the body cavity. The pronephros appears to form the swollen cephalic portion of the kidney of the adult, and the mesonephros the remainder; the so-called caudal portion, where present, being derived (?) from the postanal segmental tubes.
In some cases the cephalic portion of the kidneys is absent in the adult, which probably implies the atrophy of the pronephros; in other instances the cephalic portion of the kidneys is the only part developed. Its relation to the embryonic pronephros requires however further elucidation.
In the adult the ducts in the lower part of the kidneys lie as a rule on their outer borders, and almost invariably open into a urinary bladder, which usually opens in its turn on the urinogenital papilla immediately behind the genital pore, but in a few instances there is a common urinogenital pore.
Fig. 395. Section through the pronephros of a Trout and adjacent parts ten days before hatching.
pr.n. pronephros; po. opening of pronephros into the isolated portion of the body cavity containing the glomerulus; gl. glomerulus; ao. aorta; ch. notochord; x. subnotochordal rod; al. alimentary tract.
In most Osseous Fish there are true generative ducts continuous with the investment of the generative organs. It appears to me most probable, from the analogy of Lepidosteus, to be described in the next section, that these ducts are split off from the primitive segmental duct, and correspond with the Müllerian ducts of Elasmobranchii, etc.; though on this point we have at present no positive embryological evidence (vide general considerations at the end of the Chapter). In the female Salmon and the male and female Eel the generative products are carried to the exterior by abdominal pores. It is possible that this may represent a primitive condition, though it is more probably a case of degeneration, as is indicated by the presence of ducts in the male Salmon and in forms nearly allied to the Salmonidæ.