Donaldson, Brain and Spinal Cord of Rat. 163 



If we turn now to the preceding Table 2, we find the percentage 

 difference between the body lengths for the two sexes (as deter- 

 mined from the average of the percentage differences between the 

 five pairs ranging from 155 to 205 gms. in body weight) to be 

 1.74 per cent, in favor of the male. That is, on the average, mature 

 males of a given body weight exceed by 1.74 per cent, in body 

 length females of a like body weight. 



If now we select the body length of 103 mm., which is that for 

 the male having a body weight of 185 grams (see Table 2), and 

 consider that this body lengih is 101.74 i)er cent, of the correspond- 

 ing female body length, we find by calculation that the body length 

 of the latter is 189.7 mm., thus giving an absolute difference of 

 3.3 mm. in favor of the male. In order to determine what dift'erence 

 in the weight of the central nervous system would correspond to this 

 difierence in body length, we may refer to the preceding Table 3, 

 where the weight of the nervous system (both sexes combined) is 

 given according to the body lengths. From this table it is possible 

 to determine how much increase in the weight of the nervous system 

 corresponds to an increase of 1 mm. in body length. Taking the en- 

 tries from the body lengths of 180 to 210 mm., we obtain the fol- 

 lowing : — 



Average increase in 

 From Increase in body length the weight of the 



central nervous system 



180-190 1 mm. .0092 



191-200 1 mm. .0081 



201-210 1 mm. .0087 



Average 0087 gms. 



If the average difference in weight for 1 mm., as shown by the 

 table, is .0087 gms,, 3.3 mm. would imply an absolute difierence 

 of .02871 grams. This amount is 1.20 per cent, of the weight of 

 the nervous system for a rat 195 mm. in body length (this is the 

 mid-value between 180 mm. and 210 nun., the limits taken in the 

 foregoing table). In Table 6, of the previous jiaper, Donaldson, '08, 

 it appears as an average of all the groups taken in pairs, that 

 for rats of like body weight, but difi'ereut sex, the entire central 



