346 



COMPARATIVE ANATOMY 



a " later generation " of pronephric tubules. It appears more 

 probable, however, that this organ originates independently from 

 a part of the mesoblastic somites situated more dorsally than that 

 which gives rise to the pronephric tubules. Primitively, the 

 mesonephros is strictly metameric, owing to the fact that each of 

 its tubules corresponds to the primary channel connecting the 

 cavity of a somite with the unsegmented coelpme (Fig. 274). The 

 loss of connection between these two sections of the primary 

 ccelome results in a series of segmental nephridia, each of which 



opens into the body-cavity by a nephros- 

 tome, while at its other, or blind end, it 

 comes into connection with the prone- 

 phric duct or mesonephric duct as it 

 must now be called (Fig. 275). The 

 glomus of the pronephros is continued 

 backwards, and in the region of the 

 mesonephros breaks up into portions, or 

 glomeruli, each of which is situated in a 

 small cavity constricted off from the 

 coelome and opening into a mesonephric 

 tubule, forming what is known as a 

 Malpigliian capsule (Figs. 274, 275). 



Each mesonephric tubule, then, in 

 its primitive form, is made up of the 

 following portions (Fig. 275) : (1) a 

 funnel-shaped ciliated aperture, commu- 

 nicating with the body-cavity (nephro- 

 stome, or peritoneal funnel) ; (2) a 

 rounded mass of capillaries (glomerulus\ 

 which is situated within a cavity (Mal- 

 piyliian capsule) derived from the 

 ccelome ; and (3) a coiled glandular 

 tubule^ opening into a collecting (me- 

 sonephric) duct. Thus the mesonephros, 

 as well as the pronephros, besides its 

 main function of excreting waste pro- 

 ducts by means of the epithelial cells 

 lining the tubules, serves also to conduct water derived from the 

 blood in the glomeruli, and peritoneal fluid, from the body. 



The mesonephros is of greatest importance in the Anamnia : in 

 many Fishes it serves exclusively as a urinary organ, but in 

 Elasinobranchs and higher forms it also takes on certain relations 

 to the generative apparatus, giving rise to the rete and vasa 

 efferentia of the testis, as well as to the parorcliis or epididymis (p. 

 350), and, in Amniota, to other more or less rudimentary organs 

 of secondary importance (compare Fig. 273). Nevertheless, it may 

 still serve as the permanent urinary organ (Elasmobranchs, Am- 

 phibians), or may more or less entirely disappear as such (Amniota) ; 

 in the latter case, a third series of tubules is formed, giving rise 



FIG. 275. DIAGRAM OF THE 

 MESONEPHRIC TUBULES, 



SHOWING THEIR (SECOND- 

 ARY) CONNECTION WITH THE 

 MESONEPHRIC DUCT (SO). 



The two anterior tubules are 

 already connected with the 

 duct, while the two posterior 

 have not yet reached so far. 



ST, nephrostome; M, Mal- 

 pighian capsule with glome- 

 rulus ; DS, coiled glandular 

 tubule ; ES, terminal por- 

 tion of latter. 



