178 FOUNDATIONS OF BIOLOGY 



In the lowest Vertebrates the primitive type of kidney, or 

 PRONEPHROS as it is called, consists of a series of segmentally 

 arranged nephridia in the dorsal part of the anterior end of 

 the coelom. These, however, instead of opening independ- 

 ently to the exterior, discharge their products into a common 

 tube (PRONEPHRIC DUCT) which passes them to the outside. 

 In higher forms the pronephros disappears, and its function 

 is taken over by another series of nephridia which appear in 

 the coelom posterior to the pronephros. This series consti- 

 tutes the MESONEPHROS, and opens into the pronephric duct 

 which accordingly now is called the MESONEPHRIC DUCT. Fi- 

 nally, in still higher Vertebrates this second urinary organ is 

 replaced by a third, the kidney proper (METANEPHROS) and 

 its special duct, the URETER. Thus as we ascend the Verte- 

 brate series three distinct kidney systems appear, in each 

 case by the development and grouping of a number of ne- 

 phridia into a definitive organ. In this process the primitive 

 communication of the individual nephridia with the body 

 cavity is lost and the activity of the glandular portion in- 

 creased, until, in the higher forms, all the waste products are 

 taken solely and directly from the blood. (Fig. 97.) 



It is therefore apparent that each of the relatively large, 

 compact kidneys of the higher Vertebrates, including Man, 

 is to all intents and purposes a large group of nephridia-like 

 elements, the tubules, bound together by connective tissue 

 and covered with a protective coat. The tubules within the 

 kidney deliver the materials taken from the blood to the 

 pelvis of the kidney, from which it passes down the ureter 

 and on to the URINARY BLADDER and finally to the exterior. 

 (Fig. 98.) 



Such, in broad outline, is the historical viewpoint from 

 which the kidneys of Man must be interpreted. As a matter 

 of fact, however, the evolutionary transformation is still 



