122 RE GENERA TION 



If it be granted that the growth in a hypertrophied organ is 

 brought about by some substance that increases the function of that 

 organ, can we suppose the phenomenon of regeneration to be due 

 to similar factors ? In other words, can we reduce both phenomena 

 to the same principle ? The case is complicated by two facts that 

 may be illustrated by concrete examples. If a piece is cut from the 

 middle of the body of lumbriculus new cells are produced at both 

 ends of the piece. If we suppose the proliferation is brought about 

 by the accumulation of certain substances in the piece, we must still 

 invoke other factors to account for the differentiation of the prolifer- 

 ated material, since a head forms at one end and a tail at the other. 

 All the hypothesis can do in itself is to account for a proliferation, 

 not for the differentiation, and, both in the case of hypertrophy and in 

 that of regeneration, it is the formation of new structures that we are 

 chiefly concerned with, rather than the simple act of growth or of 

 proliferation. If a piece of a hydra is cut off, the whole piece changes 

 into the typical hydra form. Here there is no extensive process of 

 proliferation, and the change is in the old part. It seems highly 

 improbable that the production of substances in the piece could account 

 for its change of form. These examples will suffice to show that in 

 the process of regeneration it is very improbable that the change is 

 brought about by special substances that may develop or be present 

 in the part. We must suppose that during regeneration the forma- 

 tion of the typical form is not the result of a stimulus originating in a 

 chemical substance acting upon the living material, but due to changes 

 brought about directly in the living part itself. We must conclude, 

 therefore, that despite the apparently close connection between the 

 phenomena of hypertrophy of uninjured organs and of regeneration, 

 they may often involve different factors. 



If specific substances can bring about the hypertrophy of an organ, 

 it is still not clear at present whether they do so by directly causing 

 new growth, or whether their presence only stimulates the organ to 

 greater activity and the activity of the organ is the cause of its 

 growth. Since it must be supposed that in each organ a different 

 specific substance brings about its activity and the consequent hyper- 

 trophy, it seems more probable that the result is due to the activity 

 itself rather than to a stimulus from the substance. This view is fur- 

 ther supported by the fact that in the case of the muscles and of the 

 blood vessels the hypertrophy is directly connected with their use. 

 The greater use brings about a larger supply of blood, but the blood 

 is only different in amount and not in its quality. It must be con- 

 fessed that it is difficult to see how the use of a part could make its 

 growth increase, for by use the tissues break down ; and we are not 

 familiar with any other processes within the body that make for the 



