Donaldson, Nervous System of the Frog. 321 



Table 6 shows that the drying for twenty-four hours, 

 causes a marked diminution in the weight of the cord, and at 

 the same time affects the brain weight but Httle ; consequently 

 the ratio of brain weight to cord weight rises in the frogs as 

 the result of drying for this length of time. 



If the drying process is carried on for two or three days 

 the brain also loses in weight and the ratio falls again. It will 

 be noted that the " wet " body-weight is given for both groups 

 in Table 6, as this is the only basis for a fair comparison be- 

 tween the two groups. The average body-weight in Group B 

 after "drying" for 24 hours, was 176 grams. As the brain 

 has been practically unaffected by the twenty-four hours of dry- 

 ing, these frogs appear to have a high bram zvcight as compared 

 with the weight of the body when dry. The relati on is of course 

 explained by that which has just been stated. 



In determining therefore, among frogs of the same size, 

 the ratio of the weight of the brain to that of the spinal cord, 

 we find the principal source of variation to depend on the 

 amount of water taken up by the frog. 



There are doubtless other sources dependent on irregulari- 

 ties in the growth of the central nervous system and probably 

 also on modifications in the capability of the frog to take up 

 water, depending in turn on the age of the frog, the state of 

 nutrition and the season of the year ; but there is no reason to 

 think any of these conditions to be very important. 



