GROWTH OF THE CEREBRAL CORTEX 149 
opment of the cortical nerve cells by the’ method of silver im- 
pregnation of Golgi, the cortical nerve cells of the mouse have 
completed their development in respect of their attachments at 
the age of fifteen days, and the age of fifteen days is the weaning 
time of the mouse. It appears, therefore, that the completion 
of certain features of cortical development in relation to the 
Weaning time, the time when the young become independent 
of the mother, is similar in both the albino rat and the mouse. 
VIII. SUMMARY 
1. The size of the nerve cells most advanced in development 
from a fixed locality of the cerebral cortex was systematically 
measured and the developmental changes during postnatal growth 
studied on the material represented by the grains of 128 albino 
rats of different: ages. The data have been averaged for each 
brain-weight group and then corrected for the fresh condition 
of the material, using the correction-coefficients devised for this 
purpose. The results are given in tables and charts. 
2. The full size of the pyramids in the lamina pyramidalis 
(about twenty-five days in age, average of Groups XI-XIIT) 
is cell body 21 x 27 » and nucleus 19 x 21 uw and the largest size 
at birth is cell body 11x15 uw and nucleus 10x11 uw. The size 
of the ganglion cells in the lamina ganglionaris at the same stage 
(about twenty-five days in age, average of Groups XI-XIII) 
is cell body 27 x 37u and nucleus 24 x 25 yu, while the largest size 
at birth is cell body 17 x 21 uw and nucleus 14 x 16 up. 
In the full-grown albino rat (Groups XVI-XX), the average’ 
size of the pyramids is cell body 20 x 26 uw, nucleus 18 x 19 » and 
the average size of the ganglion cells is cell body 28x 38 u, 
nucleus 24 x 25 uw. 
3. The cell body and the nucleus of the pyramids attain their 
maximum size at twenty to thirty days in age (1.1 to 1.3 grams 
in brain weight). Up to the tenth day of age they retain their 
fetal morphology. After having passed the maximum at twenty 
to thirty days, they diminish in size, but the internal structure 
matures more and more as the age advances. 
