556 EZRA ALLEN 
whether such loci are constant in their appearance at fixed points 
of the system or vary with different individuals. The large num- 
ber of such loci found within small portions of the tissue at all 
ages and in each individual examined shows the necessity of using 
a considerable number of consecutive sections if one desires to 
make wide application of figures obtained from any one level. 
3. Mitosis continues for different periods after birth in the 
different levels. It ceases first in the cord. Very few dividing 
cells are to be found in the fifteen-day cord at any level, as shown 
in table 1 (p. 553). No dividing cells were found in two eighteen- 
day-old specimens, although sections widely distributed in the 
different levels respectively were examined. No mitoses were 
found in the cords of older animals. The eighteen-day period 
may then be regarded as the time when mitosis has ceased in 
this portion of the system, having ceased between this and the 
fifteen-day period. 
In the cerebellum it continues until some time between the 
twenty-second and twenty-fifth day, while in the cerebrum it 
continues still longer—to a slight degree in material about 120 
days old, further discussion of which is taken up under the topics 
which follow. 
4. Table 2 shows the distribution of dividing cells per cubic 
millimeter at three levels in the spinal cord and at one in the cere- 
bellum and cerebrum respectively. The data for these figures 
come from consecutive frontal sections at the different levels as 
fully described in the introduction (p. 552). Each number in 
the table is not an average from several different rats but is taken 
from the records of one individual. For a given age, the numbers 
italicized are from the same animal. It will be noticed that the 
figures for the cerebellum in each of the animals of the first three 
ages are not from the same, animals as those which furnished the 
figures for the other levels. This substitution is due to certain 
technical difficulties which presented themselves, such as failure 
to get good sections or to obtain true frontal sections in the 
desired locality. These three cerebella are the only ones of these 
ages upon which estimations were figured; others might have 
furnished slightly different figures. 
