20O 



BIOLOGICAL LECTURES. 



echinoderms, molluscs, annelids, and vertebrates that we 

 find regeneration taking place only by the addition of new 

 tissue at the cut ends. 



In those forms that regenerate by morpholaxis we must find 

 a theory that can account for the changing over of the old 

 piece into a new whole. To assume two formative substances 

 might account for the changes at the 

 two ends, but not for the rest of the 

 piece. 



There are still other difficulties for 

 the stuff-hypothesis. If a planarian be 

 cut in two longitudinally, each half 

 forms a new individual. Along the 

 entire length of the cut edge new tissue 

 appears. The new pharynx appears 

 along the border between the old and 

 the new tissue (Fig. 3, A, B). It lies 

 at first quite unsymmetrically in the 

 new worm, but the new side continues 

 to grow and the old side to get nar- 

 rower. The result is that although the 

 new worm is only as broad as the half from which it devel- 

 oped the pharynx lies at last in the middle plane of the body 

 (Fig. 3, C). 



Shall we assume that side-forming substances are present, 

 which, being transported laterally, bring about the development 

 of the new side ? If so, we should have to assume that at each 

 point the substance must be different from that elsewhere, 

 for a different structure is formed. I believe that few persons 

 would like to assume the responsibility for such a view. 



One further objection. If the foot of a newt is cut off, only 

 the foot develops ; if the cut is made through the forearm, 

 then the forearm and foot are both regenerated ; if the limb is 

 cut off through the humerus, then all distal to that point is 

 renewed. If we assume a leg-forming substance, the assump- 

 tion is insufficient to account for the differences in the result 

 at each level. If we assume a different kind of substance flow- 

 ing from the body for each level, then the hypothesis becomes 



FIG. 3. 



