EXTERNAL FACTORS OF REGENERATION IN ANIMALS 37 



determines the kind of regeneration in antennularia, it is possible that 

 if the regenerating piece were placed on a rotating wheel, the piece 

 might still produce a new stem at the apical end, and roots at the 

 lower end. In an experiment of this sort that I made, the pieces did 

 not, it is true, regenerate at all, but this was probably due not to the 

 change of position in regard to gravity, but to agitation of the water, 

 or to the rubbing of the cut-end against the water. It is also possible 

 that in this form the attachment of the piece at one end may be a 

 factor that may counterbalance the action of gravity. Other factors, 

 such as food, or temperature, or oxygen, appear not to determine the 

 kind of product that results, but only the rapidity with which the 

 change takes place. The salts in solution seem also to act on 

 the rate and extent of the new growth, but possibly other cases may 

 be found in which the kind of regeneration may also be affected by 

 the salts. 



It is important to find that those animals whose growth and regen- 

 eration are influenced by such external factors as light, gravity, and 

 contact are attached animals that stand in a constant relation to these 

 physical agents. They form only a very small part of the entire 

 number of animals in which regeneration takes place. Animals 

 that constantly move about are not, as a rule, influenced during 

 their growth and regeneration by gravity and contact, and under 

 natural circumstances they are always changing their position in 

 regard to these agents. Temperature, and food, and substances in 

 solution act alike on fixed and free forms, and they are, it appears, 

 both influenced in the same way by these agents. The most signifi- 

 cant fact that has been discovered in connection with the influence of 

 external factors on regeneration is that the same factors that influ- 

 ence the normal growth of the organism also affect in the same way 

 the regeneration. 



As yet an analysis of the external factors that influence growth has 

 not been made out as completely for animals as for plants, especially 

 in those cases in which the result is determined by several factors 

 at the same time. An examination of the factors that influence 

 regeneration in plants will be made in a later chapter. First, how- 

 ever, the internal factors of regeneration in animals will be consid- 

 ered. 



