SENESCENCE IN HIGHER ANIMALS AND MAN 283 



CHANGES IN WATER-CONTENT AND CHEMICAL CONSTITUTION 



From a certain stage of development on, the water-content of 

 the body undergoes in general a decrease with advancing age, as 

 many authors have shown. Davenport ('97) has found that in 

 the frog the percentage of water increases from 56 to 96 per cent 

 during the first two or three weeks after hatching, and then begins 

 to decrease. In the chick embryo and the human fetus the per- 

 centage of water decreases from an early stage. Aron ('13) has 

 compiled the data concerning the changes in water-content in man 

 and the higher animals. 



The decrease in water-content is not uniform for the different 

 organs, nor is its progress in a given organ entirely uniform in all 

 cases. The extensive investigations of Donaldson and Hatai 1 on 

 the water-content of the nervous system of the white rat show 

 that the percentage of water in this tissue changes very regularly 

 with advancing age. At birth it is about 88 per cent, at maturity 

 78 per cent, and it is altered only very slightly by nutritive con- 

 ditions and external factors. Donaldson states that it affords the 

 best index known of the age of these animals. It is probable that 

 further investigations on other mammals would give similar results 

 for the nervous system, but for various other tissues, e.g., the 

 muscles, the variation in water-content is much greater. 



It is an undoubted fact that after a certain stage the body 

 becomes more and more solid as the structural substance accumu- 

 lates. The decreasing water-content is in fact probably to some 

 extent merely another aspect of the process of structural accumula- 

 tion in the cells, although it may be in part the result of changes in 

 the aggregate condition of the colloids, as Bechhold ('12) and others 

 have suggested. 



It is impossible to consider at length the changes in chemical 

 constitution which occur with advancing age. Aron's recent com- 

 pilation of the data on the biochemistry of growth ('13) affords a 

 good survey of our present knowledge on this question. In general 

 an increase in the percentage of proteid and of inorganic substances 

 occurs, and this increase is more rapid during the earlier years of 



1 Hatai, '04; Donaldson, 'na, 'u&; Donaldson and Hatai, 'n. 



