696 ELASMOBRANCHII. 



segmental duct. Owing to the formation of the accessory tubuli 

 the segments of the mesonephros acquire a compound character. 



The third section of each tubulus becomes by continuous 

 growth, especially in the hinder segments, very bulky and 

 convoluted. 



The general character of a slightly developed segment of 

 the mesonephros at its full growth may be gathered from fig. 

 391. It commences with (i) 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 less oblique course 

 backwards, and, passing superficially to the Wolffian duct (w.d), 

 opens into (3) a Malpighian body (p. ing) at the anterior ex- 

 tremity of an isolated coil of glandular tubuli. This coil forms 

 the third section of each segment, and starts from the Mal- 

 pighian 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) accessory 

 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 with 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 con- 

 siderable number of the hindermost tubuli (ten or eleven in 

 Scyllium canicula), either in some species elongate, overlap, 

 while at the same time their openings travel backward so that 

 they eventually open by apertures (not usually so numerous as 

 the separate tubes), on nearly the same level, into the hinder- 

 most 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 per- 



