VIII PFLtJGER ON RECRESCENCE 405 



obeys the laws prescribed by it. . . . Since therefore the 

 most superficial layer of living molecules in the mutilated 

 stump, an almost imponderable minute quantity of substance, 

 produces the whole limb with mathematical necessity, very 

 much as a snowflake forms an avalanche, and since this is 

 true of all members, therefore it is not difficult to conceive 

 that the whole head and trunk can be produced from a very 

 much smaller surface, a kind of ellipsoid, if adequate nutritive 

 material is supplied to this surface." Farther on it is par- 

 ticularly insisted that the regenerating action takes pLace 

 always in a definite direction. If a nerve -trunk is cut 

 through, the surface of the portion still connected with the 

 nerve-centres exhibits an enormous regenerating activity, 

 while the other surface, together with the peripheral nerve, 

 perishes, although otherwise it is under the same conditions 

 as the former surface. The organising surfaces of a body 

 show therefore a polarisation, in that the one of their sides 

 does not exhibit the same properties as the other. The direc- 

 tion of the polarisation of the regenerating surfaces is the 

 cause of the direction of growth, and for this reason a part of 

 an animal cannot in general by growth regenerate the whole 

 animal. The peculiarity mentioned by Pfliiger of the recres- 

 cence of nerves seems to me to demand a different and obvious 

 explanation. It is easy to understand why only the central 

 portion of the divided nerve remains alive and exhibits the 

 power of recrescence, while the peripheral dies, for only the 

 former remains under the necessary influence of all the vital 

 conditions of the whole body, is still active as a nerve ; the 

 other has given up its activity as a nerve, and must therefore 



as such perish. 



Pfltiger thus supposes that just as in the embryonic 

 development of an organ its growth at a given moment takes 

 place always from the already formed " polarised " terminal 

 surface, so also in recrescence. That, he says further, in o 



