

EMBRYO AND FRAGMENTARY ORGANS. 49 



action, and to act independently of each 

 other the old force in fashioning an organ 

 on the old plan, and the new force in forming 

 some other organ according to the new type. 

 Thus at a certain stage the foetus rep- 

 resents in part the old type and in part 

 the new. 



When, in such cases, the new Specific 

 life-force seems to wholly supersede that of 

 the antecessor, a partly fashioned, incon- 

 gruous organ is dealt with, either 



By being re-formed into an organ of the 



new type ; or, 

 By being wholly or partially absorbed ; 



or, 



By its growth being simply arrested. 

 The re- formation of gill-clefts and visceral 

 i arches, visible at an early stage in the 



embryo of mammals, is an example of 

 the first process ; 



The incipient teeth, discernible at one time 

 in the embryo of the Greenland whale, 

 and afterwards wholly absorbed and 

 replaced by whalebone, illustrates the 

 second method ; and 



The third is represented by the rudimen- 

 tary upper incisor teeth in the foetus of 

 the calf, arrested in growth and covered 

 with a hard pad ; or, again, by the frag- 

 D 



