Donaldson, Growth of Central Nervous System. 347 



taken with calipers from the tip of the nose to the anus, the animal 

 lying on its side, and being gently extended to its full length. 



The measurement was recorded in millimeters as the "body 

 length." From the anus to the tip of the tail, a second measure- 

 ment was taken, which gives the length of the tail, and this was 

 recorded as "tail length." The animal was then eviscerated. 



The spinal cord was next exposed, gently raised by the filum 

 terminale, and the nerve roots clipped away (caudo-cephalad) 

 close to the cord. The division between the brain and the cord 

 was made at the tip of calamus scriptorius or just caudad to it. 

 The skull was then opened from the dorsal side, an.d the brain 

 removed. 



Immediately after removal, the brain was put in one closed 

 weighing bottle, the cord in another, and each weighed separately. 

 The meninges of both brain and cord w^re left intact. Such 

 blood as they contained, was therefore included in the weight. 



After the first weighing, the brain and cord were dried at a tem- 

 perature between 90° and 95° C. for a week or more, then re- 

 weighed, and the percentage of water determined. 



In the following pages we shall discuss only the weights of the 

 body, brain and cord, and their relations to one another, leaving 

 for later consideration, the data on body length and on the per- 

 centage of water in the brain and the spinal cord. 



The observations on the growth of the brain will be presented 

 first. 



GROWTH OF THE RAT's BRAIN. 



Table I contains 680 records (462 male, 218 female) of the 

 weight of the rat's brain. The changes in the weight of the 

 brain are most readily appreciated when the records are arranged 

 in relation to the increase in the total body weight. Such an 

 arrangement is made in chart i, plate ii, on which all the in- 

 dividual records that could be entered without confusion are shown. 

 To avoid confusion, however, it was necessary to omit a total 

 of 37 records (26 males, 11 females). The impression given 

 by this chart is therefore somewhat less strong than that war- 

 ranted by the observations. As can be seen by inspection, the 

 " scatter" of the individual entries is not very great. 



The entries on chart i suggest that the weight of the brain in the 

 male rats is heavier than in the female. To test this animals of 



