Donaldson, American and European Frogs. 139 



The brains and spinal cords of R. esculenta dried and weighed 

 at Zurich, were left in the original weighing bottles from the summer 

 of 1904 to the spring of 1907, when, after careful redrying, they 

 were weighed at the Wistar Institute in Philadelphia, 



The last series of weighings made at Philadelphia, differed 

 from those made in Zurich in 1904, by an average of plus o.i per 

 cent. The fact that there was a trifling gaiii, is probably to be 

 credited to the different balances used. But whatever the expla- 

 nation of this gain may be, it seems to show that the drying in 

 Zurich was complete, and thus to warrant the use of the values 

 for R. esculenta and R. temporaria as entered in Table 13. 



Assuming that in any given locality, the humidity of the atmos- 

 phere might be a factor influencing the amount of water in the body 

 of a frog, I made an examination of the humidity records from 

 July I to September i, 1904, taken by the weather bureaus at 

 Liverpool, Zurich and Chicago. For the data with which to do 

 this, I am indebted to the officials of the U. S. Weather Bureau, 

 whose courtesy I desire to acknowledge with thanks. 



The matter is far too complex to permit us to make here more 

 than the most general statements, but I feel justified in stating 

 that the humidity conditions at Liverpool in July, 1904, and at 

 Zurich and Chicago in August, 1904, were not unusual. Further, 

 that broadly-speaking, the humidity is greatest at Liverpool, inter- 

 mediate at Zurich, and least at Chicago. It is to be noted that 

 the percentage of water in the several species follows the inverse 

 order, being most in the Chicago specimens, where the humidity 

 is lowest, and least at Liverpool, where it is greatest; a suggestive 

 result which invites further inquiry. 



Two more comments are however desirable before leaving this 

 general topic. 



From previous studies, we should expect that the percentage 

 of water in the brain and in the spinal cord would diminish with 

 increasing age, for the measure of which we here take the body 

 weight. 



This decrease is clear and regular for the brain of R. esculenta 

 and R. temporaria, is indicated though less regular, in the case of 

 the spinal cords of these two species, but in R. pipiens is regularly 

 reversed in the case of the brain, and irregularly reversed in the 

 case of the spinal cord. This makes it highly probable that 

 some disturbing influence has modified the percentage of water in 



