2 /O A' GENERA TION 



internal one. An excellent example of an internal factor is found in 

 the interrelations of the parts 'to each other. This is shown in the 

 development of a piece of a plant in which the apical buds develop 

 faster than the proximal ones, and it appears that, in some way, the 

 development of the latter are held in check by the development of 

 the apical ones. Another case is found in the development of the 

 bilobed tail of certain fish in which particular regions are held in 

 check, while others grow at the maximum rate. 



It is a curious fact that while we can cite several kinds of external 

 influences that affect the development and the regeneration of organ- 

 isms, the only internal factors that have been discovered are the so- 

 called polarity and this interrelation of the parts. Perhaps there 

 should also be added the specific nature of certain parts, limiting 

 the possibilities of new growth in these parts, and the presence 

 of the nucleus as necessary for the growth and regeneration of the 

 organism. 



If it be admitted that the same factors that affect the growth also 

 affect in the same way the regeneration, we have made a distinct 

 advance. It is, moreover, not difficult for us to understand how this 

 is possible. If we consider first those cases in which growth takes 

 place at one or more points at which the cells are undifferentiated, 

 and compare this condition with that in regenerating animals that 

 produce new tissue by proliferation, we can picture to ourselves 

 that the same factors would act on the undifferentiated tissue in the 

 same way in both cases. This does not explain what causes the 

 organism to produce the new cells that appear over an exposed sur- 

 face, and we must search for other factors to account for the out- 

 wandering of cells, and for the local multiplication of the cells at the 

 cut-end. We find a parallel to those cases in which the growth of 

 an organism takes place throughout the whole body, in those animals 

 in which the regeneration also takes place in the old part. This com- 

 parison should not, however, be pushed too far, since, in some forms, 

 as, for example, a salamander, the growth of the animal takes place 

 throughout the body, while regeneration takes place by the prolifera- 

 tion of new material. The difference in the regenerative process in 

 a salamander and in a form like hydra is not due so much to the 

 inability of the old cells of the salamander to increase in number as 

 compared with those of hydra, but rather, it appears, to a certain 

 rigidity or stiffness of the body of the salamander that prevents the 

 rearrangement of the parts ; and the recompletion of the form takes 

 place in the direction of least resistance, i.e. at the open or cut-end 

 of the body. 



Regeneration by means of morphallaxis takes place only in those 

 forms in which the body is not mace up of a series of separated 



