CHANGES IN WEIGHT OF CENTRAL NERVOUS SYSTEM 667 
Observations on the frogs examined at Chicago, 1901-1902 
Beginning in the spring of 1901, I endeavored to examine four 
males and four females of R. pipiens each week from March 28, 
1901, to the following March, 1902. As will be seen from table 1, 
this plan met with only approximate success. 
The frogs examined in Chicago were obtained from a dealer 
who was in turn supplied from a wide range of country extending 
from southern Minnesota to Indiana. 
It was assumed at the beginning of this work that season was 
the main cause modifying the relative weight of the central 
nervous system in these frogs, and that therefore the fact that the 
frogs were taken from different localities would not materially 
modify the results. Consideration of all the facts at present in 
hand now leads me to think, on the contrary, that the relative 
weight of the nervous system is modified by station, and that in 
the case in question, the frogs from different stations were mixed 
together. As a rule however, the lots which were obtained were 
rather uniformly mixed and so, except in a few cases, no serious 
discrepancies appeared. 
As has been stated, it was planned to examine every week, four 
frogs of each sex or eight in all. From March 28, 1901, to Octo- 
ber 2, this plan was followed with moderate success, although the 
number of complete records is less than of those incomplete. 
This has come about by reason of the fact that although the full 
number of frogs was examined almost every week, nevertheless 
when the percentage of water in the brain and in the spinal cord 
was determined later, it was found in some cases that it was be- 
yond the normal limits. These I have set for the brain as 83.5— 
85.5 per cent and for the spinal cord as 79.5-81.5 per cent. When 
any record transgressed these limits for both the brain and spinal 
cord, such a record was excluded as it was assumed that deviation 
to this extent in both divisions of the central nervous system 
meant that the frog was in an abnormal state. 
Between October 2nd and the end of the year, this deviation 
in the percentage of water made it necessary to exclude all the 
records. The frogs obtained by us at this season had evidently 
