1914] Lafayette B. Mendel 175 



accomplished in this direction is indicated by the investigations of 

 Donaldson and Hatai on rats. They have found that the percent- 

 age of water in the central nervous System is a function of age and 

 under ordinary conditions, whether the animal be over size or under 

 size, well nourished or ill nourished, with a large brain or a small 

 brain, the percentage of water remains practically unmodified by 

 these conditions. The deviations in extreme malnutrition even are, 

 very slight at most and completely disappear on a return of normal 

 nutritive conditions. The percentage of water in the central nerv- 

 ous System accordingly is the best index yet available of the normal 

 process of senescence. The facts fit in with human experience in 

 showing that hardships which include under feeding need not neces- 

 sarily shorten the span of life. It is no small advantage to have a 

 dependable index of age. The experimental possibilities and de- 

 siderata in this field are far from exhausted. Chemical data are 

 needed to Supplement the histological and other analyses of age. 



Although each individual appears to strive to attain a definite 

 size it is still debated whether nature demands continuous growth or 

 to what extent remissions are detrimental or permissible. It is 

 becoming more and more evident that growth is not an entirely 

 indispensable function of living matter. Otherwise stated, the 

 failure to grow is not incompatible with life. Where are the limi- 

 tations of such situations? And above all, can life be extended by 

 the delay of growth? This is one aspect of the broad problem of 

 prolonging life artificially by altering the conditions under which 

 it goes on. Theoretically it is conceivable that growth processes, 

 like chemical reactions, should be reversible. 



The more detailed study of growth may be expected in the 

 future to bring helpful experience to bear on the manifestation of 

 pathological neoplasms which concern health so vitally. The com- 

 petition of youthful cells such as those of malignant tumors with 

 the normal cells of adult tissues frequently brings victory to the 

 younger tissue. The problems of normal development and ab- 

 normal structure are doubtless in many respects one and the same. 



" Every great advance of science opens our eyes to facts which 

 we had failed before to observe, and makes new demands on our 



