RUDIMENTARY ORGANS OF ANIMALS 139 



formerly the mesonephros. Muller's canal became 

 considerably enlarged ; it formed the vagina, the 

 uterus, and the Fallopian tubes ; at the upper end 

 it was connected with the hydatid, a vestige of the 

 mesonephros. 



It is plain then that the genital-urinary apparatus 

 of the female comprises some organs of which the 

 functions remain unchanged : the ovaries, the per- 

 manent kidney, the Fallopian tubes, the uterus, the 

 vagina, and the ureter, and some rudimentary organs, 

 vestiges of what were once active organs ; the 

 paraovarium, the paraophoron, hydatid and Weber's 

 organ. 



The complicated development of this system 

 becomes clear if a careful study is made of the 

 history of the genito-urinary apparatus of the entire 

 series of vertebrates. 



It appears that the various phases through which 

 the embryos of the higher vertebrates pass are 

 stages similar to those which may be observed in 

 the adult lower vertebrates. 



The principle of recapitulation, that the embry- 

 onic stages of higher animals recapitulate successive 

 stages attained by the adults of lower animals, 

 receives a full corroboration from the facts we have 

 been displaying. 



Amphioxus, for instance, remains still at the 

 pronephric stage : fish as a rule have a mesonephric 

 or permanent kidney. Some lizards (Lacerta) up 

 to the age of two years make use of the mesone- 



