THE PHYSIOLOGY OF REGENERATION 343 



cent of water in the tissues increases rapidly when growth is at 

 its height, and diminishes when the latter declines. The parallelism 

 between growth and regeneration, previously discussed on the 

 basis of the periodic change in their intensity, finds further con- 

 firmation in the similarity of the curves of the water content in 

 growth and in regeneration. 



We are thus led to the generalization that regeneration is the 

 renewal of growth; but in referring regeneration to the category 

 of growth phenomena nothing is gained in the way of a better 

 understanding of its physiology. Growth is one of the fundamen- 

 tal functions in the living world, but its nature is very difficult 

 to analyze, as the multiplicity of definitions it has already received 

 indicates. Growth never occurs apart from other functions and, 

 therefore, cannot be studied entirely by itself; hence these differ- 

 ent points of view : that growth is increase in mass, or is continuous 

 change of form, or, finally, is increase in mass accompanied by 

 changes in form. The common meaning attached to growth, 

 however, has made the first definition the more popular. But 

 if regeneration and growth are to be homologized, it may be well 

 to bear in mind that the regenerating organism, at least in the 

 case of Podarke, diminishes in bulk. 



These worms were becoming smaller while the new tails 

 were growing and that happened whether or not the animals 

 were fed. It would be of little consequence if the general- 

 ization, that regeneration is renewed growth, caused only an 

 inconsistency in definitions; its real significance lies in the 

 implied presence of similar factors in both growth and regen- 

 eration. In all studies of embryonic growth it has been shown 

 that at certain stages the body increases through the absorp- 

 tion of water from the surrounding medium. But considering 

 critically the water changes in regeneration it cannot be over- 

 looked that the rise and fall of the per cent of water re- 

 sults from an internal regulation, or a series of occurrences 

 tending to restore normal relations. The increase of the per cent 

 of water here is not due to imbibition of water from the outside, for 

 as a matter of fact the absolute quantity of water in the organism 

 is continually diminishing. It is, furthermore, significant that 



