REGENERA TION. 1 99 



ation is influenced by the orientation of the pieces with respect 

 to gravity ; in others to light ; in others to contact. In those 

 cases in which gravity is the determining factor, we could 

 readily imagine that the transportation of head-forming or tail- 

 forming substances is brought about by the action of gravity. 

 In the other cases in which light or contact has an influence 

 on the regenerating part, it is not easy to see how a stimulus 

 of this sort could bring about the transportation of material, 

 although we can readily imagine that either factor acting 

 locally might cause the production of some substance which, in 

 turn, might act on the new tissues. 



These illustrations would seem, in several cases at least, to 

 harmonize well with the stuff-hypothesis, less well, perhaps, 

 with the idea of its transportation. On the other hand, one 

 does not have to look very far to find other cases to which 

 the stuff-transportation hypothesis does not apply, or applies 

 badly. 



In the first place, in many of the lower animals regeneration 

 does not take place by the development of new tissues, but by 

 a remoulding morpholaxis of the entire piece into a new 

 form. This side of the question has been almost entirely neg- 

 lected by those who have proposed hypotheses of regeneration, 

 and yet it seems to me that just here we find some of the most 

 important phenomena. A protozoon cut into pieces makes as 

 many new individuals of small size as there are nucleated 

 pieces. The size of the new individual is, within certain limits, 

 in proportion to the size of the fragment, and it develops 

 regenerates not by forming new material at the cut surfaces, 

 but by remoulding the entire piece into the characteristic 

 form. If a piece is cut from a hydra, it bends together and 

 makes a sphere or cylinder out of which a new hydra is formed. 

 There is no evidence of the formation of new tissue, unless the 

 tentacles are formed in that way ; but in other hydroids, viz., 

 Tubularia and Parypha, even the tentacles are known to 

 develop out of the old cells. In planarians a small amount of 

 new tissue forms at the cut ends, and out of this a new head 

 and a new tail develop, but the old piece changes over to form 

 the body of the new individual. It is in the higher forms alone 



