Excretory and Reproductive Systems 



The excretory and reproductive systems, functionally very far 

 apart, are closely related anatomically. This relation evidently results 

 from the fact that the two systems have a common need — namely, pas- 

 sages leading to posterior exits from the body. Certain ducts are 

 urinogenital. 



Excretory (Renal) Organs 



These organs consist of systems of minute excretory tubules ag- 

 gregated to form a pair of kidneys which are always situated on the 

 dorsal wall of the abdominal cavity and close to either side of the 

 median plane. The renal tubules are so closely adjacent to small blood- 

 vessels as to allow waste substances to diffuse from the blood into the 

 cavities of the tubules. 



Systems of renal tubules may develop (1) at the extreme anterior 

 end of the embryonic coelom, (2) at a more posterior position, or (3) 

 at a still more posterior location and in a manner differing from that in 

 which the more anterior kidneys develop. According to the position and 

 the manner of embryonic origin, three types of kidney are recognized 

 (Fig. 77): (1) pronephros, (2) mesonephros, (3) metanephros. In 

 fishes and amphibians the definitive adult kidney may be (but rarely) 

 a pronephros, but in most cases it is regarded as being a mesonephros. 

 In adult reptiles, birds, and mammals, the functional kidney is a meta- 

 nephros, but in the embryos there appear a vestigial pronephros and a 

 temporarily functional mesonephros, the anterior part of which persists 

 in the male adult as a system of tubules (the epididymis) transporting 

 sperm from the testis into the sperm duct (Fig. 78C). 



Reproductive "Glands" 



The gonads, ovary and testis, develop as a pair of organs situated 

 on the dorsal body-wall in the anterior region of the embryonic ab- 



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