i 

 CHAPTER XV 



THE KIDNEYS AND THE EEPKODUCTIVB ORGANS BREEDING 



The kidneys and the reproductive organs are so intimately con- 

 nected that it is necessary to deal with them together. Both 

 organs are specialised portions of the coelom and its epithelial 

 lining. The kidneys are essentially a series of tubular and at 

 first segmentally-disposed outgrowths from the coelom (urocoeles) 

 which acquire a connexion with the exterior, while the gonads 

 have their origin from local modifications of the coelomic epi- 

 thelium. At a very early embryonic stage each lateral half 

 of the coelom presents three well-marked divisions: (1) a series 

 of dorsal portions (" myocoeles "), the cavities of the myotomes or 

 muscle-segments ; (2) a longitudinally continuous unsegmented 

 portion extending round the alimentary canal, the " ventral 

 coelom " ; and (3) a series of intermediate tubular portions or 

 " nephrotomes," each of which leads from a myocoele to the ventral 

 coelom (Fig. 229, A). The essential components of the kidneys, 

 the urocoeles or renal tubules, are derived from the nephrotomes. 

 In its typical condition each kidney consists of three portions, 

 which, in accordance with their embryological and evolutionary 

 sequence, are termed the " pronephros," the " mesonephros,'' and the 

 " metanephros." The pronephros, the larval or provisional kidney, 

 is formed from a limited number of the nephrotomes immediately 

 behind the head. From each nephrotome a hollow tubular out- 

 growth is formed, which grows towards the lateral surface of the 

 body, and then unites with its fellows of the same side to form 

 a main longitudinal duct — the " archinephric " or " pronephric 

 duct" (Fig. 229, A, Fig. 230, A). This duct grows backwards 

 until it opens into the cloaca.' At the same time the nephro- 



' It is probable that the archinephric duct is derived from the embryonio 



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