CHANGES IN WEIGHT OF CENTRAL NERVOUS SYSTEM 691 
As this gradually becomes exhausted, feeding is resumed and with 
the advance of the season and the increase in food, the frog not 
only grows, but restores these reserves and prepares for the next 
period of hibernation. 
This review of the feeding habits of the frog serves to empha- 
size the fact that the conditions for nutrition in the early part of 
the season are different from those to be found later, and in so 
far might be responsible for the peculiarities in the relative growth 
of the central nervous system which we have observed. 
In addition there are several considerations which have a very 
immediate bearing on the foregoing results, and especially on their 
variability. It must always be remembered that we are working 
with an animal in which the regulation of both general and rela- 
tive growth is poor: an animal very responsive to the influence 
of external conditions—one that canbe chilled or warmed, dried 
or made moist, fed abundantly or left without food for long 
periods. 
Thus, in a poor season, 7.e., poor in insects, or in the water con- 
ditions, the frog may not exhibit its usual increase in size, may not 
store its full food reserve for the first half of the year to follow and 
so not only grow poorly in general, but also not be able to exhibit 
the usual relative growth of the central nervous system during 
the season which follows. 
It is hardly necessary to elaborate these relations, enough hay- 
ing been said to indicate why frogs taken at the same date and 
in the same locality, may exhibit wide differences in the relative 
weight of the central nervous system. What we find in the case 
of any frog probably depends in large measure on the external 
conditions to which that individual has been subjected, not only 
during the season in which it was caught, but also during the sea- 
son which preceded. If one turn back therefore to table 1, 
it appears that in the Chicago series the minimal values of C vary 
irregularly from month to month; suggesting that some of the 
individuals grew very little—as this would be the readiest explan- 
ation of the absence of systematic changes—while the maximal 
values show more consistent changes, tending to follow the mean. 
On the other hand, in the case of both the Minnesota and 
Brandywine frogs, both the minimal and maximal values tend to 
