OLFACTORY BULBS OF THE ALBINO RAT 251 
2. All layers of the olfactory bulb are about equally concerned 
in the increase in size or in the arrest of development of the 
bulb. 
3. The small cells of the molecular layer show a larger amount 
of cytoplasm in large bulbs than in small ones. 
4. The number of small cells in the molecular layer, appar- 
ently, is not correlated either with age of the rat or size of the 
bulb. The entire computed number for a small, medium, and 
large bulb was found to be approximately 1,000,000 cells + 2 
per cent. 
5. The mitral cells of small bulbs are smaller, on the average, 
than those of large bulbs. 
6. Within the limits here taken the number of mitral cells is 
not affected by the age or the size of the bulb. 
7. There seems to be some variation between litters in the 
number of the mitral cells. The average number of mitral 
cells for 13 bulbs was 76,750, the lowest number being 70,625, 
and the highest 83,974. The standard deviation o is 4564 and 
the probable error of the mean + 855. 
8. When members of the same litter are compared, bulbs 
stunted by a defective diet or enlarged by exercise show practi- 
cally the same number of mitral cells as do their controls. The 
mean difference is —1.8 per cent for the tests. 
9. The olfactory bulb size is correlated with cell size and not 
with cell number. 
VII. LITERATURE CITED 
ALLEN, E. 1912 The cessation of mitosis in the central nervous system of 
the albino rat. Jour. Comp. Neur., vol. 22, no. 6, pp. 547-568. 
Boucuton, T. H. 1906 The increase in the number and size of the medul- 
lated fibers in the oculomotor nerve of the white rat and of the cat 
at different ages. Jour. Comp. Neur., vol. 16, pp. 153-165. 
Donaupson, H. H. 1894 Preliminary observations on some changes caused 
in the nervous tissues by reagents commonly employed to harden 
them. Jour. Morph., vol. 9, pp. 123-166. 
Dunn, E. H. 1912 The influence of age, sex, weight, and relationship upon the 
number of medullated nerve fibers and on the size of the largest fibers 
in the ventral root of the second cervical nerve of the albino rat. 
Jour. Comp. Neur., vol. 22, pp. 131-157. 
