24 SENESCENCE AND REJUVENESCENCE 



most cases simply in the postulation of a complex invisible morpho- 

 logical structure of one kind or another as the basis of the visible 

 structure which develops ; with such theories the problem of struc- 

 ture remains and is less accessible than before. 



The experiments mentioned above demonstrate that such a com- 

 plex invisible structure is quite unnecessary as a basis for visible 

 morphogenesis. In the case of many of the artificial structures the 

 determining conditions are not at all complex and the process is 

 readily analyzed. It is certainly not too much to say that these 

 experiments in the production of form constitute a real and impor- 

 tant step toward the solution of the problem of organic morpho- 

 genesis. From them we can at least see the possibility and even 

 the probability of reducing the problem of structure to other and 

 simpler terms, that is to say, terms of dynamic processes, and that 

 must be reckoned as no slight advance. 



But the colloid substratum in the organism is of importance in 

 many other ways. The capacity of many of the organic colloids 

 for taking up water is of very great importance in determining and 

 maintaining the water content of organisms. A certain water 

 content is indispensable for the normal activity of every organism 

 and every part. We know, moreover, that various inorganic 

 substances alter the capacity of colloids to take up or hold water 

 and evidence is rapidly accumulating that many normal and patho- 

 logical variations in water-content are at least in part determined 

 by changes in the colloids which in turn result from changes in the 

 content of certain inorganic salts and other substances. 



The content and distribution of the salts themselves is also 

 influenced by the colloids. Changes in the colloids alter the salt- 

 content, as regards either amount or kind. The permeability of 

 colloid membranes to the ions of salts and other substances and the 

 changes which they undergo with changes in conditions is believed 

 by many to be of great importance for many of the processes of 

 life. Authorities are not fully agreed as to the part played by 

 colloid surface membranes in organisms. While the theory of semi- 

 permeable membranes and of changes in permeability has been 

 very widely accepted, there are some facts which indicate that 

 other factors besides membranes are concerned in the penetration 



