CHAPTER IX 

 THE URO-GENITAL SYSTEM 



" We do not draw conclusions with our eyes, but with 

 our reasoning powers, and if the whole of the rest 

 of living nature proclaims with one accord from all 

 sides the evolution of the world of organisms, we can- 

 not assume that the process stopped short of Man. 

 But it follows also that the factors which brought 

 about the development of Man from his Simian an- 

 cestry must be the same as those which have brought 

 about the whole of evolution." 



AUGUST WEISMANN, The Evolution Theory, 

 Authorized translation. Vol. II, p. 393. 



THE two systems included under this compound name are 

 those concerned with the very diverse functions of the elimi- 

 nation of liquid waste and the formation of new individuals. 

 They are, however, closely associated topographically "and 

 usually possess certain parts in common, so that they belong 

 together anatomically, though not physiologically. They pos- 

 sess also important relations to the body cavity or ccelom 

 (more strictly, metaccele), and as the latter is in by no means 

 a primitive condition in either Amphioxus or the cyclostomes, 

 recourse must be had to early embryonic stages and also to 

 invertebrates in order to reconstruct the early period of the 

 history of these organs, a knowledge necessary for the ex- 

 planation of many of the existing relationships. To begin 

 with, let us suppose an animal built on the plan of a gastrula, 

 but with a space left between the endoderm and the ectoderm 

 (Fig. 105, A; also Plate I). Such an animal consists of two 

 tubes, one inside the other, and two cavities. The two tubes 

 are, of course, alimentary canal and body wall, and the two 

 cavities are the digestive cavity (gastrocwle) and the primary 

 body cavity (protoccele) . Within this protoccele are contained 

 two sets of organs, each opening to the exterior, excretory tu- 



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