288 HELEN DEAN KING 



ing lengths of time, it seemed possible that the age of the solu- 

 tion might be a factor that would appreciably affect its swelling 

 action on brain tissue. To test this assumption the following 

 series of experiments was made. 



Series 2. In this series, and in all others that were made sub- 

 sequently, brains from rats of the following ages were used : new- 

 born, 10, 20, 40, 50, 70, 100 and approximately 200 days old. 

 Six rats of each age were used, the animals being taken from the 

 same litter in order to avoid any possible variation in brain 

 structure that might be characteristic of difTerent litters. Three 

 brains of each age were placed in a neutralized solution of 4 per 

 cent formaldehyde that had been standing for five months; the 

 remaining three brains of the same age were put into a neutral- 

 ized solution that was made when the experiments were started. 

 As all the animals were killed within a period of two weeks, the 

 second solution was comparatively fresh when used on the final 

 set of brains. 



Each brain was put into 40 cc. of the solution and kept at 

 laboratory temperature. The weighings were made on the first, 

 third, and seventh days of preservation, also at the end of the 

 second, third and tenth weeks. The specimens were then heat 

 dried for one week and again weighed in order that the percent- 

 age of solids might be obtained. 



Table 2 gives the data for the brains kept in the 'old' solution. 



The data obtained in the experiments in which the brains were 

 kept in the freshly made solution are given in table 3. 



The results of these experiments are much more uniform than 

 those obtained in the first series. The maximum amount of 

 swelling was reached in all brains by the third day, and then 

 there was a very gradual decline in weight until, at the end of 

 ten weeks, the percentage weight increase was from 1 to 12 per 

 cent below the maximum; in both sets of experiments the great- 

 est loss in weight took place in the brains of new-born rats. 



If we compare the data in table 2 with that in table 3, it is 

 seen that the age of the solution used has a very marked effect 

 on the amount of swelling in the brains at all ages. The old 



