THEORIES OF REGENERATION 2? I 



parts. This kind of regeneration occurs in those organisms in 

 which the normal growth consists only in the enlargement of a sys- 

 tem of organs already present. A piece of an animal of this sort 

 usually contains the elements of each kind of organ, and from these 

 the new parts are produced, both by proliferation at the cut-ends and 

 by the enlargement of the parts that are present in the piece. In 

 forms with separate segments we find, in some cases, resemblances 

 between normal growth and regeneration, as shown, for example, 

 in the earthworm. There is present in the young worm a region in 

 front of the last segment, or, rather, a part of this segment, from 

 which new segments are formed. In the regeneration of the posterior 

 end a knob of new tissue is formed, and out of this a few segments 

 develop, the last one having a growing region similar to that in the 

 young worm. The subsequent stages in the regeneration involve 

 the formation of new segments from the last one, as in the young 

 worm. There is no such growing zone at the anterior end of the 

 young worm, and none is formed in the regeneration of an anterior 

 end, so that only the segments that are first laid down in the new 

 part are present in the new anterior end. 



An interesting comparison may be made between the phenome- 

 non of growth and that of contraction and expansion of the proto- 

 plasm. The bending of heliotropic organisms toward or away from 

 the light, and the similar bending of negatively stereotropic forms 

 away from contact with a solid body, are supposed to be phenomena 

 of growth, and resemble in many ways the phenomenon of contrac- 

 tion. In a plant that bends toward the light, it is found that the 

 most obvious change involves the amount of water on the two sides 

 of the stem, and this is most probably connected with a fundamental 

 structural change in the protoplasm, that is too subtile for further 

 analysis. In the regeneration of some forms it is found that they re- 

 spond in the same way to light. While it cannot be demonstrated 

 that these phenomena really depend on processes of contraction 

 and of expansion, the results are nevertheless suggestive from this 

 point of view. Furthermore, I think, one cannot study the regenera- 

 tion of such forms as planarians, hydras, stentors, etc., without being 

 struck by the apparent resemblance of the change in form that they 

 undergo to a process of expansion. The idea of the expansion of 

 a viscid body carries with it, of course, the idea of tension within the 

 parts, and the return to the former condition is brought about by a 

 release from the tension and a return to a more stable condition. 

 If by the intercalation of new material the extended condition is 

 fixed, a new state of equilibrium will be established. 



It has been already pointed out that in a piece of a plant suspended 

 in a moist atmosphere, the apical buds are those that first develop, 



