EXCRETORY ORGANS. 601 



The chief structural peculiarity of the pronephros is the absence 

 from it of Malpighian bodies with the same relations as those in the 

 meso- and metanephros ; unless the structures found in Myxine are 

 to be regarded as such. Functionally the place of such Malpighian 

 bodies is taken by the vascular peritoneal ridge spoken of in the 

 previous pages as the glomerulus. 



That this body is really related functionally to the pronephros appears 

 to be indicated (1) by its constant occurrence with the pronephros and its 

 position opposite the peritoneal openings of this body ; (2) by its atrophy 

 at the same time as the pronephros ; (3) by its enclosure together with 

 the pronephridian stoma in a special compartment of the body cavity in 

 Teleostei and Ganoids, and its partial enclosure in such a compartment in 

 Amphibia. 



The pronephros atrophies more or less completely in most types, 

 though it probably persists for life in the Teleostei and Ganoids, and 

 in some members of the former group it perhaps forms the sole adult 

 organ of excretion. 



The cause of its atrophy may perhaps be related to the fact that it is 

 situated in the pericardial region of the body cavity, the dorsal part of 

 which is aborted on the formation of a closed pericardium ; and its preserva- 

 tion in Teleostei and Ganoids may on this view be due to the fact that in 

 these types its peritoneal funnel and its glomerulus are early isolated in a 

 special cavity. 



Mesonephros. The mesonephros is in all instances composed of a 

 series of tubules (segmental tubes) which are developed inde- 

 pendently of the segmental duct. Each tubule is typically formed of 

 (1) a peritoneal funnel opening into (2) a Malpighian body, from 

 which there proceeds (3) a coiled glandular tube, finally opening 

 by (4) a collecting tube into the segmental duct, which constitutes 

 the primitive duct for the mesonephros as well as for the pronephros. 



The development of the mesonephridian tubules is subject to con- 

 siderable variations. 



(1) They may be formed as differentiations of the intermediate 

 cell mass, and be from the first provided with a lumen, opening into 

 the body cavity, and directly derived from the section of the body- 

 cavity present in the intermediate cell mass ; the peritoneal funnels 

 often persisting for life (Elasmobranchii). 



(2) They may be formed as solid cords either attached to or inde- 

 pendent of the peritoneal epithelium, which after first becoming inde- 

 pendent of the peritoneal epithelium subsequently send downwards 

 a process, which unites with it and forms a peritoneal funnel, which 

 may or may not persist (Acipenser, Amphibia). 



(3) They may be formed as in the last case, but acquire no 

 secondary connection with the peritoneal epithelium (Teleostei, 

 Amniota). In connection with the original attachment to the 

 peritoneal epithelium, a true peritoneal funnel may however be 

 developed (Aves, Lacertilia). 



