MITOSIS IN CENTRAL NERVOUS SYSTEM 553 
(myelospongium) which has a columnar and radial disposition 
and shows three layers: (a) the innermost or germinal zone; 
(b) a middle nuclear or mantle zone crowded with daughter cells 
which have migrated from the actively mitotic germinal layer; 
and (c) an outer nuclear-free reticular or boundary zone. (His 
89 and ’04; Hardesty ’04; Bryce ’08). The seat of mitotic 
activity is confined at first to the germinal zone but later spreads 
to the mantle layer (Hamilton ’01; Hardesty ’04). Activity 
continues in these two layers with increasing age, but becomes 
more rapid in the latter (the middle nuclear or mantle). Hamil- 
ton (’01), writing of the spinal cord of the albino rat, states: “As 
TABLE 1 
Giving the percentage of ependymal mitoses in the spinal*cord of the albino rat from 
one day to fifteen days old, based upon the number of mitoses in ten sections from 
the cervical, thoracic and lumbar levels respectively, excepting that in the fifteen- 
day rat twenty sections at each level were used. 
AGE | TOTAL NUMBER OF MITOSES EPENDYMAL MITOSES mrtg ee, 
days I ie 7 yer 
1 | 47 4 8.5 
4 96 0 0.0 
115 6 oe 
2 22 2 9.0 
15 4 1 25.0 
Morals... 284 13 | 4.5 
The percentage of 13 to 284 is 4.5, the percentage of the ependymal mitoses to 
the total number of mitoses during the period from 1 day to 15 days. 
development proceeds, there is a relative increase of extra-ven- 
tricular mitoses, so that by the end of the first day after birth 
they are greatly in the majority.”” My preparations from the 
one-day cord of the same animal confirm this observation. At 
this age the mitoses are distributed in the ependyma, in the white 
and gray regions, in the fiber tracts, in the nerve roots, in the 
enveloping membranes and among the spinal ganglion cells. 
With advancing age the distribution appears as shown in table 1, 
the data for which were obtained by adding together the number 
of mitoses found in ten sections taken from the cervical, thoracic 
