CONCERNING TADPOLES 205 



when the whole of this mass of food yolk has been eaten. 

 The embryo is now provided with external gills ; but 

 these, instead of serving as breathing organs, as in all the 

 cases we have so far examined, take on the work of feeding, 

 absorbing a nutritive fluid formed by the maternal 

 uterus, a method of feeding met with elsewhere'only among 



LARViE OF THE SNAKE-LIKE ICHTHYOPHIS. 



One of these has been much enlarged. Note the great development of the external 

 gills, and the huge size of the yolk-sac. 



certain fishes, and furnishing a wonderful instance of the 

 manner in which organs developed for one purpose may in 

 course of time take on another and totally distinct office. 

 But even now we have not reached the end of this re- 

 markable history. For if embryos in the second stage of 

 their growth be artificially released from the uterus and 

 placed in a tank of water, they will thrive as well as the 



