.574 ELA8M0BRANCHII. 



The general character of a slightly developed segment of the 

 mesonephros at its full growth may be gathered from fig. 391. It 

 commences with (1) a peritoneal opening, somewhat oval in form 

 (st.o) and leading directly into (2) a narrow tube, the segmental tube, 

 which takes a more or le-^s oblique course backwards, and, passing 

 superficially to the Wolffian duct (w.d), opens into (3) a Malpighian 

 body (p.nig) at the anterior extremity of an isolated coil of glandular 

 tubuli. This coil forms the third section of each segment, and 

 starts from the Malpighian body. It consists of a considerable 

 number of rather definite convolutions, and after uniting with tubuli 

 from one, two, or more (according to the size of the segment) acces- 

 sory Malpighian bodies {a.mg) smaller than the one into which the 

 segmental tube falls, eventually opens by (4) a narrowish collecting 

 tube into the Wolffian duct at the posterior end of the segment. 

 Each segment is probably completely isolated from the adjoining 

 segments, and never has more than one peritoneal funnel and one 

 communication icith the Wolffian duct. 



Up to this time there has been no distinction between the 

 anterior and posterior tubuli of the mesonephros, which alike open 

 into the Wolffian duct. The collecting tubes of a considerable num- 

 ber of the hindermost tubuli (ten or eleven in Scy Ilium canicula), 

 either in some species elocgate, overlap, and eventually open by 

 apertures (not usually so numerous as the separate tubes), on nearly 

 the same level, into the hindermost section of the Wolffian duct in 

 the female, or into the urinogenital cloaca, formed by the coalesced 

 terminal parts of the Wolffian ducts, in the male ; or in other species 

 become modified, by a peculiar process of splitting from the Wolffian 

 duct, so as to pour their secretion into a single duct on each side, 

 which opens in a position corresponding with the numerous ducts 

 of the other species (fig. 392). In both cases the modified posterior 

 kidney-segments are probably equivalent to the permanent kidney 

 or metanephros of the amniotic Vertebrates, and for this reason the 

 numerous collecting tubes or single collecting tube, as the case may 

 be, will be spoken of as ureters. The anterior tubuli of the primitive 

 excretory organ retain their early relati(jn to the Wolffian duct, and 

 form the permanent Wolffian body or mesonephros. 



The originally separate terminal extremities of the Wolffian ducts 

 always coalesce, and form a urinal cloaca, opening by a single aper- 

 ture, situated at the extremity of the median papilla behind the anus. 

 Some of the peritoneal openings of the segmental tubes in Scyllium, 

 or in other cases all the openings, become obliterated. 



In the male the anterior segmental tubes undergo remarkable 

 modifications, and become connected with the testes. Branches 

 appear to grow from the first three or four or more of them (though 

 probably not from their peritoneal openings), which pass to the base 

 of the testis, and there uniting into a longitudinal canal, form a 

 network, and receive the secretion of the testicular ampullae (fig. 

 39.3, nt). These ducts, the vasa efferentia, carry the semen to the 



