Figures 17 to 22 are from the cerebrum of rats of different ages, showing por- 
tions of the zones bordering the ectal lateral wall of the lateral ventricle at about 
the locality marked mn? in figure 10, with the exception of figure 19, which is from 
the cortex. 
Figs. 17 and 18 One-day cerebrum; small portions of germinal layer separated 
from each other by only a few cells. > 800. 
Fig. 19 One-day cerebrum; small portion of the cortical region of the same 
section as figures 17 and 18. > 800. 
Fig. 20 Six-day cerebrum; adjacent portions of germinal and mantle layers. 
x 800. 
Fig. 21 Twenty-five-day cerebrum; two dividing cells in mantle layer, the 
lower at the edge of a group of cells (mn? in fig. 10). (See p. 555.) X 800. 
Fig. 22 120-day cerebrum; germinal and mantle layers and two cells showing 
cytoplasm in the first layer of differentiated tissue. At this age the ependymal 
cells are clearly differentiated; they began to appear in the twenty-five-day 
material, as shown in figure 21. The reduction in the number of cells in the two 
inner layers is to be noticed. > 800. 
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