286 



HELEN DEAN KING 



sex, and with this difference in size is found a corresponding dif- 

 ference in brain weight since, as shown by Donaldson ('09), the 

 size of the brain is correlated with the size of the animal, not 

 with its age. 



The times when the various weighings were made in this 

 series of experiments, together with the average percentage weight 

 increase for each group of three brains are given in table 1. 



T.\BLE 1 



Percentage weight increase in rats' brains kept for eighteen weeks in a stock solu- 

 tion of 4 per cent formaldehyde neutralized with NaCOi {averages for three brains 

 at each age) 



1 Maximum weight increase. 



The results of this series of experiments show that a 4 per 

 cent solution of formaldehyde causes a pronounced swelling in 

 the brains of rats of all ages. The maximum weight increase is 

 reached, in most cases, during the first week, and there is then 



