GROWTH OF THE CEREBRAL CORTEX 249 
Guinea-pig. I have had the opportunity at The Wistar In- 
stitute to examine the sections of the guinea-pig brains prepared 
by Allen (04) for her study on the myelination of the nervous 
system of that animal. The sections were cut in series in the 
frontal plane from material fixed in Miiller’s fluid, imbedded in 
celloidin and stained by Weigert’s method for the myelin sheaths. 
The thickness of the cerebral cortex in the adult guinea-pig 
(body weight, 618 grams; brain weight not recorded) is on the 
average 1.90 mm. (1.80 mm., 1.88 mm., and 2.01 mm., respec- 
tively, at the localities corresponding to localities VI, VII, and 
VIII examined by me on the frontal section of the Albino brain 
at the level of the commissura anterior). The corresponding 
measurements at birth (body weight, 108 grams) are 1.71 mm. 
(and 1.51 mm., 1.75 mm., and 1.86 mm., respectively) and those 
at thirty-five days (body weight, 250 grams) are 1.85 mm. (and 
1.77 mm., 1.86 mm., and 1.92 mm., respectively). So, from 
birth on to the maturity, the cortical thickness has on the aver- 
age increased only 11 per cent. According to Allen, the guinea- 
pig at birth is covered with hair, has complete muscular devel- 
opment, and is almost independent of the mother, the central 
nervous system being practically completely myelinated, whereas, 
by contrast, the albino rat is born quite naked, extremely help- 
less and undeveloped, and myelination in the brain has not be- 
gun. The guinea-pig is psychically mature soon after birth 
(three days after birth); the degree of development of the cen- 
tral nervous system of the new-born guinea-pig corresponds to 
that of the albino rat at twenty-three to twenty-seven days or 
its period of first psychical maturity. A new-born guinea-pig 
is found to have a cerebral cortex in which the myelination is 
going on. 
Comparing the sections from the guinea-pig brain with those 
from the albino rat brain, it appears that the new-born guinea-pig 
corresponds to the albino rat of about ten days in cortical thick- 
ness, but seems to be older when judged by the myelination of 
the cortex. This coincides with observation that the guinea- 
pig is, almost from the start, relatively independent of the 
mother. 
