208 FOUNDATIONS OF BIOLOGY 



neys as the testes in the male, do not enter into communica- 

 tion with a set of nephridia of the kidneys (mesonephros) ; 

 probably because the eggs are too large to pass through the 

 tubules. Instead, what appears to be the coelomic opening, 

 or NEPHROSTOME, of a single nephridium on either side (which 

 fails, so to speak, to enter the kidney complex) enlarges and 

 becomes the funnel which connects up with a new duct open- 

 ing into the cloaca. Thus there arises from the female urinary 

 system a pair of entirely distinct OVIDUCTS. An egg, liberated 

 from the ovary into the coelom, finds its way into one of the 

 oviducts and descends directly to the outside, or into an en- 

 largement (uterus) of the terminal portion of the duct where 

 development proceeds until birth occurs. (Fig. 97, D.) 



The female reproductive system, though derived from the 

 mesonephric system, has become entirely independent of it. 

 Accordingly the disappearance of the mesonephros and duct 

 in higher Vertebrates, when it is replaced by the metanephros 

 and the ureter as the functional urinary system, has little 

 effect on the female reproductive system. As a matter of 

 fact the abandoned mesonephros and duct degenerate and 

 disappear in the female, while in the male the mesonephric 

 duct remains and becomes completely appropriated by the 

 reproductive system. The sperm now pass directly into the 

 former mesonephric duct, which thereby becomes solely a 

 sperm duct. Such is the historical origin of the foundations 

 of the reproductive system as it occurs in the Reptiles, Birds, 

 and Mammals. Naturally each of these groups, building on 

 this foundation, has developed modifications and additions 

 demanded by its special lines of evolution; (Fig. 97, E, F.) 



