248 CAROLINE M. HOLT 
Table 3 gives the results of the counts of the mitral cells of 
fourteen olfactory bulbs, arranged according to bulb weight. 
It is obvious that there is no correlation between bulb size and 
the number of the mitral cells or between age—within the limits 
taken—and number of cells. The numbers range from 70,625 
to 83,974 with an average of 76,750 cells for 13 bulbs, X, being 
omitted from the average. I am inclined to think 83,974 cells 
is too high a count for G; and that still closer enumeration might 
yield a lower number. <A recount was made for C; counting 
every fifth section, as this was a somewhat irregular bulb and 
it was thought that might account for the variation of this bulb 
from the rest of the litter. But the recount gave practically 
the original number. 
Table 4 which is arranged by litters indicates a striking 
agreement between the members of the same litter. With the 
exception of litter C, in which C; falls 12 per cent below the con- 
trol in number of mitral cells, there is extremely little difference 
between test and control bulbs of the same litter. So we find 
that whether the bulb has been stunted by a defective diet, or 
enlarged by exercise, the number of mitral cells is practically 
constant for any given litter. The factors which have brought 
about a change in size of the olfactory bulbs have failed to affect 
the number of mitral cells, at least in the gray layer. The test 
and control counts are, with one exception, extremely close. 
This is a fresh example of the similarity in structure among 
members of the same litter—a relation which is continually 
appearing in the study of this animal. 
3. Study of the small cells in the gray layer 
The sections of bulbs Mu, Ms, Xi, and X, are all series in which 
a count was made of the small cells of the gray layer in the ante- 
rior portion of the bulb as well as of the mitral cells. Assuming 
that the relation between the number of mitral cells in two given 
sections of a bulb would be the same as that between the small 
cells of the gray layer in these same regions, computation was 
made of the total number of small cells in the gray layer of Mag, 
