250 CAROLINE M. HOLT 
the lateral ventricles until the end of the second year. Hatai 
observed an increase in number of cells in the spinal ganglia, 
corresponding to increase in age but this increase was attributed 
in part, at least, to failure to count all the ganglion cells in 
very small animals. Ranson (’06) in a study of the second cervi- 
eal nerve found no correlation between the number of cells and 
the number of myelinated fibers, neither did he find the number 
of cells to vary with the age of the rat. 
The results of the present investigation of the number of cells 
in the olfactory bulb help to confirm the impression that the 
number of cells in the central nervous system becomes fixed at 
an early age so that after the first three or four weeks at least, 
there is no material change in the cell number. 
This study also gives us reason to believe that the number 
of small and of mitral cells in the gray layer of the olfactory bulb 
is very nearly the same for all individuals with especially close 
agreement between individuals of the same litter. Itseems fairly 
evident that while external conditions may modify to a con- 
siderable extent the size of the brain of the albino rat and espe- 
cially the size of the olfactory bulbs, the only effect is upon the 
relative development of the individual cells. The number of 
cells remains the same. The fibers have yet to be examined. 
It is important to bear in mind in a determination of this 
sort—e.g., the number of mitral cells—that a fixed number, 
in the physical sense, is not to be expected, for all organisms are 
normally variable in all of their parts, variability being an es- 
sential character for living things; so the number which is ob- 
tained gives a mean value which we take to be characteristic 
for the species under the present conditions, but around which 
equally characteristic variations also occur. 
V1. CONCLUSIONS 
1. For bulbs of different ages and sizes, the regions anterior 
to the cerebrum, which are commonly considered the bulbs, are 
not strictly homologous, since, in the brains of young or stunted 
rats, a larger proportion of the bulb lies beneath the cerebrum 
than in the ease of the better developed brains. 
