202 CAROLINE M. HOLT 
4. Series E: Rats in revolving cages for ninety-eight to one hundred 
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Possible effect: oni fertility. 222: .ehGk sete ee one eee 225 
Effect on brain and olfactory bulbs........................ 226 
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I. INTRODUCTION 
The various members of the mammalian series show con- 
siderable variation in the relative development of all parts of 
the central nervous system, but probably no part of the encepha- 
lon shows so great a degree of variability as does the rhinencepha- 
lon. Of this portion of the brain, the olfactory bulbs are, with- 
out doubt, the most variable in size. Thus we have the very 
large bulbs of the opossum and the ant-eater; the almost rudi- 
mentary bulbs of the ape and of man; extreme reduction of 
these organs in the Cetacea, with their complete disappearance 
in the dolphin. Not only do we find variation in size of the 
olfactory bulbs among the different orders of mammals, but 
we find that there is a considerable degree of variability within 
each order and even among the members of the same species. 
This variation in size and weight of the olfactory bulbs within 
a species is well illustrated by observations upon the rats in the 
colony of The Wistar Institute. The domesticated albino rats 
exhibit a considerable range in the development of this part 
of the brain. But while we find an appreciable difference in 
the bulb size of rats of different litters even under like environ- 
mental conditions, the individuals of a given litter usually show 
a more uniform development of the olfactory system. Some 
wild Norway rats examined at The Wistar Institute a few years 
ago had olfactory bulbs heavier in proportion to total brain 
weight than the bulbs of the albino. In the course of the present 
study, observations made upon some thirty wild Norway rats 
caught at different places in Philadelphia suggested that this 
difference between the two strains is not a constant one, for, 
