PERCENTAGE OF WATER 143 



time required for the successive steps is determined by the inten- 

 sity of the growth process characteristic for each species. (Rub- 

 ner '08 and '08a). 



CONCLUSIONS 



1. In the albino rat between birth and maturity, the per- 

 centage of water in the brain diminishes from 87.8 to 77.5 and in 

 the spinal cord from 85.6 to 68.0. Table 1. 



2. The progressive diminution of the percentage of water is a 

 function of age and is not significantly modified by any conditions 

 to which the animals have been thus far experimentally subjected. 



3. The diminution in the percentage of water is most rapid 

 during the first twenty-five days of life; the period at which the 

 central nervous system is growing most actively. 



4. The maturing of the axone substance is characterized by a 

 greater diminution in the percentage of water than is the matur- 

 ing of the gray substance. 



5. Medullation begins when the percentage of water in the 

 brain and cord has diminished to about 85.3 per cent (second 

 day in the spinal cord; eleventh day, in the brain). 



6. The process of medullation itself as indicated by the forma- 

 tion of the medullary sheaths, is not a controlling factor in reduc- 

 ing the percentage of water in the central nervous system. 



7. The range and course of the diminution of the percentage 

 of water in the brain are similar in man and in the albino rat. 

 The rapidity of change agrees with the intensity of the growth 

 processes in each of the two species, and is therefore about thirty 

 times more rapid in the rat than in man. This point has not 

 been tested for the spinal cord. 



8. It is probable that the same limits in the percentage of 

 water and the same course of diminution will be found to occur 

 in other mammals. 



9. The progressive diminution of the percentage of water in 

 the central nervous system with advancing age, is to be regarded 

 as an index of fundamental chemical processes, which take place 

 in the more stable constituents the nerve cells. 



These processes are but little modified by changes in the environ- 



