CHANGES IN COMPOSITION OF BODY OP RAT 35 



We infer from the above that the bodies of all the mammals, 

 whether carnivorous or herbivorous, during their growth pass 

 through identical phases of chemical alterations, though the 

 time required to complete the life cycle may be widely different 

 in different species. The corresponding phases of chemical 

 alteration appear to follow the water content of the body. Con- 

 sequently we may conclude that the percentage of water is an 

 indicator of corresponding phases of chemical alteration of 

 the mammalian bodies, while neither the calendar age nor the 

 body weight of the animals can be used for the same purpose. 



It has been noted by Donaldson ('06) that the .normal span 

 of hmnan life is about thirty times that of the albino rat. This 

 conclusion was based on the observation that man requires 

 thirty times as many days to double his birth-weight as does 

 the rat (p. 26, table 3) and on the assumption that the average 

 maxunum age of man is 90 years, while that of the albino rat 

 is three years. Donaldson noted further ('08) that the graph 

 of growth for the brain in weight is similar both in man and in 

 the rat if the age intervals of man are reduced one-thirtieth, 

 and further the brain weight of adult man (1400 grams) is re- 

 duced to the adult brain weight of the rat (2 grams). This re- 

 sult shows then that the two main phases of growth (postnatal 

 cell division and the swelling process) in these two forms are 

 similar at corresponding ages. 



In his third communication Donaldson ('10) shows not only 

 that these growth phases agree in man and in the rat, but also 

 the percentage of water in the brain agrees in the two species 

 at equivalent ages. From this we infer that the percentage of 

 water in the brain may be taken as the mesaure of the equiva- 

 lent ages of man and of the rat. 



Very recently Mayer and Schaeffer ('14) studied the relation 

 between the amount of water and of various lipoids in several 

 visceral organs in various mammals and reached the conclusion 

 that a certain definite parallelism exists between the degree of 

 imbibition of water and the ratio "cholesterine — fatty acids," or 

 "cholesterine — lipoid phosphorus." 



