SGO-BUGTS AND SPfiRM-DUCm. 403 



called egg-ducts {oviductus, or tuhoe faZlopice). In the 

 male sex these tubes convey the sperm-cells from the testes, 

 and hence they are called sperm-ducts (spermddiLctua, or 

 vasa deferentia). 



The original, genetic condition of these two outlets 

 is exactly the same in Man as in all higher Vertebrates, 

 while in most Invertebrates it is entirely different; for 

 while in the latter the sexual ducts develop directly from 

 the sexual glands, or from the external skin, or from the in- 

 testinal canal, in Vertebrates an organ-system is employed 

 for the conveyance of the sexual products ; one which origin- 

 ally had a very different significance and function — the 

 kidney system, or urinary organs. The original, primary func- 

 tion of these organs is simply to eliminate useless matter 

 from the body in a liquid form. The liquid product of this 

 secretion is called the urine, and is discharged either directly 

 through the external skin, or through the last section of the 

 intestine. The tube-shaped " urinary ducts " only second- 

 arily absorb the sexual products also and convey them out ; 

 they thus become "urogenital dticts" (ductus urogenitates). 

 This remarkable secondary combination of the urinary and 

 the sexual organs into a common " urogenital apparatus," or 

 ** urogenital system," is highly characteristic of the higher 

 Vertebrates. In the lowest of these it is, however, wanting, 

 while, on the other hand, it is foimd in the higher Ringed 

 Worms (Annelida). To estimate this rightly, we must first 

 glance at the comparative economy of the urinary organs 

 as a whole. 



The kidney system or urinary system (aystema uro- 

 poeticum) is one of the earliest and most important organ- 

 systems in the differentiated animal body, as has already 



