554 EZRA ALLEN 
and lumbar regions of the cord respectively, with one exception, 
viz., that twenty sections instead of ten were used from each 
region as a basis for the figures for the fifteen-day specimen. 
Hamilton (’01), from observations on twenty-five consecutive 
sections from the lumbar cord of one new-born rat, concludes 
that the greatest number of mitoses is in the anterior gray column. 
This relationship apparently does not persist during the later 
stages of mitotic activity, as shown by my sections. Further- 
more the animals which I have studied show great individual 
variation at the same age. In addition, the same animal varies 
greatly in its number of mitoses in series of sections taken at only 
short distances apart in the same approximate level. 
The ependyma of the cerebellum shows very few mitoses in the 
one-day-old animal. © In the cortex they are abundant; there are 
few in the intermediate tissue. This condition prevails through- 
out the length of the organ. By the age of twelve days cell 
division is confined to the cortical area and does not reappear in 
other portions. 
The cerebrum shows a distribution the reverse of that found 
in the other two divisions. While at birth it is diffuse, the great- 
est amount of activity is seen in the germinal and mantle layers 
about the lateral ventricles (fig. 3). The activity external to 
these layers grows less until at the age of twelve days there is 
scarcely any cell division elsewhere. Furthermore, the activity 
about the lateral ventricles is not equally distributed. It is least 
on the ental lateral surface and the ventral portion of the ectal 
wall; greatest along the roof and dorsal portion of the ectal lateral 
wall, where the mantle layer is widest (fig. 7). This distribution is 
to be seen in the animal one day old at whatever level of the ven- 
tricle we may section. In the older animals the region of activity 
becomes limited to the small portion of the ectal lateral wall lying 
a little ventrad from the point of union of this wall and the roof. 
(See further discussion of this zone under the topic Structural 
Correlations). 
2. A comparison of sections from the same rat taken at differ- 
ent levels shows that the rate of mitosis is not equal throughout 
the length of the central nervous system. The cord is the region 
