PERCENTAGE OF WATER 123 



For the brain, the records show in both sexes ranges in the per- 

 centage of water in the different age groups as follows :- 



AGE PERCENTAGES 



0-10 days total range 3 units 86-89 



10-20 days total range 5 units 82-87 



From 20 to 100 days the range diminishes, and after this latter 

 age it does not amount to more than one unit. The ranges for 

 the spinal cord are less than those for the brain. 



It will naturally be asked whether among individuals belonging 

 to the same litter, reared under similar conditions and killed at 

 exactly the same age there is any difference in the percentage 

 of water between those having relatively heavy brains and spinal 

 cords and those in which these organs are relatively light. This 

 question seems to be answered in the negative by the result of 

 25 pairs of observations recently made. 



The figures are as follows : — 



PERCENT PERCENT 



OF WATER OF WATER 



Heavy brains 78.651 72.436 Heavy cords 



Light brains 78.649 72.465 Light cords 



In both instances as is seen, the differences found are too small 

 to be significant. It may be added that the weight of the light 

 brains was about 96 per cent that of the heavy, and similarly 

 the weight of the corresponding light spinal cords about 93 per 

 cent. Such differences as we find therefore among specimens of 

 the same age must depend on some other cause than the individ- 

 ual variations in the weight of the central nervous system. 



I feel sure that the irregularities seen in the curve for the cord, 

 chart 2, 95-115 days, are purely incidental and would not appear 

 on repeating the observations. 



At the same time it is seen that the percentage of water in the 

 female spinal cord after the period of rapid growth, is in general 

 a trifle higher than in the male. This is an unexpected result. 

 The mean difference as determined from those entries in table 2 

 where there are data for both sexes at a given age (i. e., up to 

 230-240 days) is 0.36 per cent. At the moment this difference is 

 most readily explained as one effect of the passive lengthening 



