URINOGENITAL ORGANS 445 



further back possibly along the whole length of the trunk, and 

 then became reduced owing to the development of another set of 

 urinary tubules constituting the mesonephros. 



Mesonephros. 



The mesonephros or Wolffian lody has usually a much greater 

 extent than the pronephros : it is originally strictly segmental, in 

 correspondence with the mode of development of the mesodermic 

 somites. The tubules of which it is composed correspond with 

 those parts of the mesoderm described above as the nephrotomes l 

 (p. 441), which become separated from the somites but retain their 

 connection with the general body-cavity, into which each of them 

 opens by a ciliated nephrostome, 2 the other end becoming connected 

 with the pronephric duct, which thus now serves as a mesonephric 

 duct. 



The tubules then increase in length, each becoming coiled into 

 an S-shape and differentiated into several portions, the middle one 

 expanding to form a vesicle (Bowman's capsule), into which a 

 glomerulus formed on a branch of the aorta becomes invaginated, 

 the whole constituting a Malpighian capsule (Fig. 337). It will be 

 noticed that the glomeruli are here formed in a different manner 

 to those of the pronephros, which arise on the wall of the general 

 body -cavity. 



The further development of the mesonephros varies greatly in 

 different Vertebrates : in many Fishes it serves exclusively as a 

 urinary organ, but in Plagiostomes and higher forms it also 

 takes on relations to the generative apparatus, giving rise to the 

 rete and vasa efferentia of the testis, as well as to part of the 

 epididymis or parorchis, and, in Amniota, to other more or 

 less vestigial organs of secondary importance (parovarium, paro- 

 sphoron, hydatid of Morgagni, paradidymis, cf. Fig. 339). Never- 

 theless, it may still serve as the permanent urinary organ 

 (Elasmobranchs, Amphibians), or may more or less entirely dis- 

 appear as such (Amniota) ; in the latter case, a third series of 

 tubules is formed, giving rise to a metanephros, or hind-kidney, 

 with which is connected a metanephric duct or ureter. 3 



1 These, as already mentioned, are primarily hollow coelomic canals, but in 

 Sauropsida and Mammalia they are at first solid and become hollowed secondarily. 



' The nephrostomes may be wanting, and this is especially the case in the 

 higher types, owing to the early separation of the tubules from the ccelome as 

 well as from the somites. 



3 The fact that in certain Lizards and Mammals larger or smaller portions 

 of the mesonephros retain their urinary function for a time in post-embryonic 

 stages, indicates that some of the ancestors of the Amniota retained the meso- 

 nephros throughout life as a functional excretory organ, before the complete 

 differentiation of the metanephros. On the other hand, the mesonephros is so 

 much reduced in the embryos of certain Mammals (e.g. Mouse) that it cannot 

 have any importance as a urinary organ, the excretory function here being 

 probably performed by means of the allantoic vessels and possibly those of the 

 umbilical cord. 



