MITOSIS IN CENTRAL NERVOUS SYSTEM 955 
of the least and the cerebellum the seat of the greatest activity, 
while the cerebrum in that portion where cell division is most 
rapid exhibits an activity somewhat greater than that of the cord. 
Table 2 (p. 557) contains figures illustrating these different rates. 
In addition, mitotic activity is unequal at different levels of the 
cord and cerebrum. Table 2 brings out this relativity for the 
cord. The rate of cell division is seen to be lowest in the thoracic 
and highest on the whole in the cervical portion. In the latter 
it rises on the sixth day to a rate equal to the most rapid exhibited 
by the cerebrum (on the fourth day), and more than twice as 
rapid as that of the cerebrum at six days. From a comparison 
of numbers of dividing cells at the three levels of the cerebrum, 
it is found that the greatest number is in the region of sections 
which pass through the optic chiasma. 
This unequal longitudinal distribution is still further illustrated 
in a small way within the limits of each level, as shown by the 
markedly greater number of mitoses to be found in two or more 
consecutive sections of a series, this greater number standing out 
prominently from the relatively smaller number in the sections 
immediately preceding and following. Three illustrations fol- 
low, chosen from many which show the same relationship. The 
figures representing the number of mitotic cells in each section of 
_ the series run as follows: (1) from the cerebrum—4, 5, 7, 9, 11, 16, 
14, 21, 3, 9, 3, 9, 3, 4; and (2) from the cervical cord 4, 0, 2, 
3, 5, 8, 1, 4, 2. The numbers italicized in each series mark 
this grouping of mitotic activity, which may be termed a ‘locus’ 
of activity. Similar loci.appear in the cerebellum. (3) A bril- 
liant illustration presented itself in the cerebrum of a twenty- 
five-day specimen, an age when the mantle layer is not so crowded 
with nuclei as it is in younger material. This illustration will 
also show the tendency of cells to form groups and to retain a 
group identity. In this instance three well-marked groups of 
closely-packed, densely-staining nuclei were found, each well 
differentiated from the surrounding nuclei, and each showing 
mitoses. (See mn'~mn? in fig. 10; enlarged view of one group is 
to be seen in fig. 21). These groups extended through three con- 
secutive sections of 8 micra. I have not endeavored to ascertain 
