SUPRARENAL BODIES. 463 



of tubules are differentiated from the mesoblast, and, acquiring a con- 

 nection with the segmental duct, constitute the mesonephros, or mid- 

 kidney. The tubules arise usually, though not invariably, nearer the 

 posterior end of the body than the pronephros, and are formed from the 

 portion of the mesoblast which connects the muscle segment and the 

 lateral plate (see Fig. 196). Below the Amniota the mesonephros forms 

 the permanent excretory organ. In higher forms another series of 

 nephridial tubules arises still further back in the body, and forms the 

 metanephros, or permanent kidney. The mesonephric and metanephric 

 tubules resemble each other closely, but the relation of the former to the 

 pronephros is still a debated point. When fully developed, a mesone- 

 phric tubule consists of — (1) an internal ciliated funnel (nephrostome), 

 which opens into the body cavity, but is only rarely represented ; (2) a 

 small cavity (Malpighian capsule) believed by some to be derived from 

 the ccelom, and containing a mass of capillaries which project into the 

 cavity of the tubule ; and (3) a coiled tube in part excretory, in part 

 a conducting canal for the waste filtered from the blood. The meta- 

 nephric tubules have a quite similar structure, but the nephrostome is 

 never present. 



The segmental or pronephric duct on each side is, at 

 any rate in some of the lower Vertebrates, divided into two 

 ducts, the Miillerian duct and the mesonephric or Wolffian 

 duct. In the Amniota the origin of the Miillerian duct 

 is still somewhat obscure. It becomes the genital duct or 

 oviduct of the female, while in the male the Wolffian duct 

 becomes the genital duct or vas deferens. 



The ureters or ducts from the persistent functional 

 kidneys are either the original archinephric or segmental 

 ducts (e.g. in Cyclostomata), or the Wolffian ducts (in 

 Amphibians), or special posterior derivations of the latter. 



Suprarenal bodies. — These are found in most Vertebrates near the 

 reproductive organs and kidneys. They are not known in Cyclostomes 

 or Dipnoi, but seem to increase in importance as we ascend the series. 

 Typically, each shows a distinction into a cortical and a medullary zone. 

 It is usually asserted that these two areas have a different origin, the 

 medullary region being derived from the sympathetic nervous system, 

 the cortex from the Wolffian body, or even from the most anterior part 

 of the germinal epithelium. On the other hand, some investigators 

 derive the medulla from metamorphosed cortical cells. There is much 

 evidence (morphological and physiological) that the suprarenals of 

 Elasmobranchs correspond to the medullary part in Mammals, while 

 the interrenals of Elasmobranchs and the suprarenals of Teleosts and 

 Ganoids correspond to the cortical portion in Mammals. 



With regard to function, there is even more uncertainty. The supra- 

 renal bodies are relatively very large in embiyonic life, but fail to 

 maintain their primitively rapid rate of growth. It has been suggested 

 that they assist in breaking down or disposing of waste pigment. 



