CHAPTER XV 
THE KIDNEYS AND THE REPRODUCTIVE 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 seementally-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- 
1 It is probable that the archinephric duct is derived from the embryonic 
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