Fig. 397. Transverse section through the anterior part of an Acipenser embryo. (After Salensky.)
Rf. medullary groove; Mp. medullary plate; Wg. segmental duct; Ch. notochord; En. hypoblast; Sgp. mesoblastic somite; Sp. parietal part of mesoblastic plate.
In both forms the ducts unite behind with the cloaca, and a pronephros of the Teleostean type appears to be developed. This gland is provided with but one[255] peritoneal opening, which together with the glomerulus belonging to it becomes encapsuled in a special section of the body cavity. The opening of the pronephros of Acipenser into this cavity is shewn in [fig. 398], pr.n. At this early stage of Acipenser (larva of 5 mm.) I could find no glomerulus.
The mesonephros is formed some distance behind, and some time after the pronephros, both in Acipenser and Lepidosteus, so that in the larvæ of both these genera the pronephros is for a considerable period the only excretory organ. In Lepidosteus especially the development of the mesonephros occurs very late.
The development of the mesonephros has not been worked out in Lepidosteus, but in Acipenser the anterior segmental tubes become first established as (I believe) solid cords of cells, attached at one extremity to the peritoneal epithelium on each side of the insertion of the mesentery, and extending upwards and outwards round the segmental duct[256]. The posterior segmental tubes arise later than the anterior, and (as far as can be determined from the sections in my possession) they are formed independently of the peritoneal epithelium, on the dorsal side of the segmental duct.
Fig. 398. Transverse section through the region of the stomach of a larva of Acipenser 5 mm. in length.
st. epithelium of stomach; yk. yolk; ch. notochord, below which is a subnotochordal rod; pr.n. pronephros; ao. aorta; mp. muscle-plate formed of large cells, the outer parts of which are differentiated into contractile fibres; sp.c. spinal cord; b.c. body cavity.
In later stages (larvæ of 7-10 mm.) the anterior segmental tubes gradually lose their attachment to the peritoneal epithelium. The extremity near the peritoneal epithelium forms a Malpighian body, and the other end unites with the segmental duct. At a still later stage wide peritoneal funnels are established, for at any rate a considerable number of the tubes, leading from the body cavity to the Malpighian bodies. These funnels have been noticed by Fürbringer, Salensky and myself, but their mode of development has not, so far as I know, been made out. The funnels appear to be no longer present in the adult. The development of the Müllerian ducts has not been worked out.
Dipnoi. The excretory system of the Dipnoi is only known in the adult, but though in some respects intermediate in character between that of the Ganoidei and Amphibia, it resembles that of the Ganoidei in the important feature of the Müllerian ducts serving as genital ducts in both sexes.
Amphibia. In Amphibia (Götte, Fürbringer) the development of the excretory system commences, as in Teleostei, by the formation of the segmental duct from a groove formed by a fold of the somatic layer of the peritoneal epithelium, near the dorsal border of the body cavity ([fig. 399], u). The anterior end of the groove is placed immediately behind the branchial region. Its posterior part soon becomes converted into a canal by a constriction which commences a short way from the front end of the groove, and thence extends backwards. This canal at first ends blindly close to the cloaca, into which however it soon opens.