38 FROM FISH TO PHILOSOPHER 



that both urine and reproductive cells get out of the 

 body one way or another in all extant forms— or they 

 would not be extant; and perhaps also in the fact that 

 this historic conflict between the kidney and the repro- 

 ductive organs has had the most profound eflFects on 

 moral values and psychopathology, though space pro- 

 hibits an excursus into these topics. 



In the higher animals the excretory system consists of 

 two nearly identical kidneys located at the back of the 

 abdominal cavity. In man each kidney contains nearly 

 a million nephrons, each consisting of a glomerulus and 

 tubule, these nephrons differing from each other only in 

 minor and probably not significant details of structure. 

 The nephrons drain into a treelike system of collecting 

 ducts by which the urine is conveyed to the renal (the 

 proper adjective pertaining to the kidney) pelvis and 

 thence by way of the ureter to the bladder. 



The embryologist Ernst Haeckel once said that 'on- 

 togeny recapitulates phylogeny'— meaning that during 

 their embryonic development organisms recapitulate 

 their evolutionary history. Haeckel's aphorism must be 

 accepted with many reservations, because during evolu- 

 tion patterns of embryonic development have been 

 changed as much as the structure of adult organisms, 

 but the aphorism is true in many important respects. 

 And in several ways the evolutionary history of the kid- 

 ney is recapitulated in its development. Most notably, 

 in the embryos of all reptiles, birds, and mammals, two 

 separate but abortive kidneys develop before the adult 

 kidney is formed, a fact that recapitulates the long con- 

 flict between the testis and the renal tubules for the 

 archinephric duct. The first embryonic kidney, or 'pro- 

 nephros,' forms an archinephric duct by the fusion of 

 a few of its tubules— just as this duct must have been 

 formed in the ostracoderms— but in only a few of 

 the lowest fishes do the pronephros and archinephric 

 duct continue to function in the adult: in the hagfish 



