SUPRARENAL BODIES, 491 



At a late period in those types in which the pronephros is a functional 

 larval organ, but much earlier in the higher Vertebrates, another series 

 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. 235). 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 (i) 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 coelom, 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 mela- 

 nephric tubules have a quite similar structure, but the nephrostome 

 is never present. 



In all Vertebrates the primitive nephridia open into a 

 pair of longitudinal ducts, developed like the nephridia as 

 special portions of the ccelom. These ducts open into the 

 end of the gut. According to their connections with the 

 nephridia these longitudinal ducts are called pronephric, 

 archinephric, or mesonephric ducts, and they are also called 

 segmental ducts. In Elasmobranch fishes a Miillerian duct 

 is separated off from in front backwards from the 

 longitudinal duct and forms the oviduct of the female, a 

 rudiment in the male. After the separation of the Miillerian 

 duct, the longitudinal duct (now called mesonephric or 

 Wolffian) forms in the male the vas deferens and also 

 receives the tubes from the permanent kidney. In the 

 female the Wolffian duct has this last function. In general 

 it may be said that the original longitudinal duct becomes 

 the vas deferens in the male Vertebrate, and that another 

 duct the Miillerian whose development is obscure except 

 in Elasmobranchs, forms the oviduct. 



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, 



