GROWTH OF THE CEREBRAL CORTEX 269 
carbol-thionine and acid fuchsin. <A part of his results on the 
Purkinje cells is here quoted: 
The Purkinje cells are easily distinguishable at birth along the 
inner boundary of the molecular layer by their relatively large size and 
lightly staining nucleus. These cells measure 12x 7m and nuclei 
8x 6.3 u. During the first week, there is great increase in size of 
both nucleus and cytoplasm. The main bulk of the latter is at the 
ectal pole and from it several fine processes radiate into the molecular 
layer. At eight days the cells measure 18x 12m and nuclei 10x 8 u 
to 12x9u. At-eight to ten days there is definite change in form by 
the elongation of the cytoplasm of the ectal pole to form the main 
dendrite, the previously existing fine processes becoming its branches. 
At the same time all the dendries become arranged in one plane, and 
this plane is parallel to sections directed across the folia. Nissl gran- 
ules appear in the cytoplasm at eight to ten days. The arrangement 
of Purkinje cells changes with the increase in the surface area of the 
cortex. At birth they are arranged in two to three irregular rows; 
at three days in one to two irregular rows, and at five days in one 
continuous row. As growth of the cortex continues, the space inter- 
vening between the Purkinje cells becomes greater. Some nuclei reach 
their maximum size of 12x 9 u at eight days, while the cell bodies 
usually continue to grow, reaching a maximum size of 24x 19m at 
twenty days. The dendrites reach the outer limiting membrane 
when all the outer granule cel’s have migrated (twenty-one to twenty- 
five days), and continue to develop new branches until a much later 
period as is shown by a comparison of cells from a 31 day with cells 
from a 110—day cerebellum. 
From this it is plain that the Purkinje cells (cell bodies) of the 
albino rat cerebellum have also reached full size at about the 
weaning time (twenty days of age). 
From the foregoing, we see that the functional cortical cells 
both in the cerebrum and in the cerebellum reach their full size 
at an early age—before the weaning time—and though they 
continue to mature after that they change only slightly in size, 
sometimes even diminishing. Thus the cortical nerve elements 
are all precocious in their growth, which is nearly complete 
when the young become independent of the mother and their 
education begins. Addison (’11) has stated also that the de- 
velopment of motor control in the young rat is closely correlated 
with the completion of the cerebellum and the rat attains its 
full motor control when the cerebellum has attained structural 
