194 JOURNAL OF COMPARATIVE NEUROLOGY. 
Professor DonaLpson, I undertook a series of experiments to 
test this hypothesis. 
Technique. 
For the purpose of this investigation young rats were 
especially suited, because of their immature condition at birth. 
According to a recent observation by Watson (36) (1903) in 
this laboratory, the nervous system of the new born white rat 
does not contain a single medullated nerve fiber. During the 
first few weeks the increase in the weight of the brain is very 
rapid. For example, calculating froma series of records on 
the brain weight of the white rat collected by Professor Don- 
ALDSON, the youngest rat used in the investigation gained 500% 
in brain weight (from .2 g. to 1.0 g.) during the forty days fol- 
lowing the operation. This increased weight is due in large 
part to the formation of new nerve fibers. We should expect 
that tissue undergoing such rapid development would react to 
an injury quite differently from that more nearly adult. Pre- 
vious investigations have, however, been made largely on adult 
animals, or on those in a much later phase of development. 
Those used for this study were at the time of the operation 
aged respectively, No. 1, 21 days; No. 2,7 days; \No:iaye 
days; No. 4, 0.5 days (12 hours). 
The operation was very simple. A sharp-pointed thin 
bladed scalpel, 1 mm. in breadth, was passed through the soft 
skull and run 2 or 3 millimeters into the brain substance, far 
enough to cut through the corpus callosum. The wound was 
made in each case parallel to, and about 1 mm. to the left of 
the great longitudinal sinus and 2 mm. in front of the lateral 
sinus. These sinuses, which mark the medial and posterior 
boundaries of the cerebral hemispheres, are clearly visible 
through the membranous skull. They served as guides in 
making the operation, enabling one to cut into very nearly the 
same region in each brain. The operation consisted of a single 
stab, so producing a wound the shape and size of the end of 
the knife. 
The usual aseptic precautions were observed; the instru- 
