GROWTH OF THE CEREBRAL CORTEX 135 
fetal form of the cells,*? represented by a relatively large, round 
nucleus thinly enveloped by a small amount of homogeneous 
eytoplasm and with processes from both poles. The Nissl 
bodies begin to appear first in a brain weighing 0.8 gram (VIII), 
showing first in a part of cytoplasm adjoining the nucleus at the 
apical pole and forming the so-called ‘Kernkappe.’ The cyto- 
plasm matures rapidly in structure as the brain weight increases 
from 0.8 to 1.2 grams. As the measurements show, the nucleus 
attains nearly the full size when the brain weighs 0.95 gram (10 
days), but at that phase the cytoplasm has not yet been fully 
developed. It is meagre in mass, enveloping the nucleus thinly, 
Fig. 5 Showing the cerebral cortex proper at the locality II (fig. 2, Sugita, 
‘17 a) ona fetal brain of the albino rat. Body weight about 1.0 gram, body length 
(neck-rump) about 19.5 mm., eighteenth day of gestation. Magnification of 
about 500 diameters, measured directly on the slide. 
the Nissl bodies not being yet fully differentiated, but only sug- 
gesting the ‘Kernkappe.’ The cell continues to grow very 
slowly up to a brain weight of 1.1 to 1.3 grams or about 20 to 30 
days in age. Then, as the age advances, the sizes of both the cell 
body and of the nucleus slowly diminish, while within the cyto- 
plasm the differentiation of the Nissl bodies progresses. As the 
differentiation progresses, the general tone of color of the section 
’'The form of the fetal nerve cells from the locality II of the cerebral cortex 
of the albino rat is shown diagrammatically in figure 5, which was taken from 
an Albino fetus of 1.0 gram in body weight, 19.5 mm. in body length at the 
eighteenth day of gestation. The cortex proper, not regarding the transitional 
layer, consists of four or five rows of cells with scanty cytoplasm. The aver- 
age diameter of the nucleus is about 5 to 7 micra on the slide and the thickness 
of the cortex at this age is about 0.06 mm. on the slide. 
