22 C. M. JACKSON 



In regard to the brain, it may be noted that the maximum 

 postnatal relative size occurs, not at birth, but a short time 

 later. Using the data derived from Donaldson's formula express- 

 ing the brain weight in terms of (gross) body weight ('08, table 

 1), calculations show that with a body weight of 5 grams (ap- 

 proximately that at birth) the brain weight forms 4.6 per oent 

 of the body weight. This increases to 6.7 per cent at a body 

 weight of 15 grams. From this (apparent) maximum, the relative 

 size of the brain decreases to 5.0 per cent at 25 grams, 2.7 per 

 per cent at 55 grams, 1.5 per cent at 115 grams, 1.2 per cent 

 at 155 grams, 0.9 per cent at 205 grams, and 0.62 per cent at 

 315 grams. In this case, the sexes are not separated, although 

 the male brain is relatively slightly heavier than that of the 

 female. 



The foregoing figures for the relative size of the brain, as de- 

 rived from Donaldson's formula, agree fairly well with the present 

 data grouped according to age. We find (combining Donaldson's 

 data with my own) that in 92 newborn albino rats (56 males, 

 36 females) the brain averaged 4.8 per cent of the (gross) body 

 weight; in 22 (10 males, 12 females) at seven days, 6.4 per cent; 

 in 31 (28 males, 3 females) at three weeks, 5.6 percent; in 30 

 (27 males, 3 females) at six weeks, 3.1 per cent. Only scattering 

 observations are available at the later stages. It therefore ap- 

 pears probable that the brain reaches its maximum relative size 

 about the second or third week, when the body weight is between 

 10 and 15 grams. 



The combined data for the brain are sufficient for estimation 

 of variability and correlation with body weight only at birth, 

 three weeks, and six weeks. The coefficient of variation for the 

 brain was found to be at birth, male, 13.9; female, 9.1, total 

 (sexes together) 12.2. The coefficient of variation for the cor- 

 responding (gross) body weights was 12.8. At three weeks the 

 coefficient of variation of the brain, for males only, was 6.8, 

 which, excepting the eyeballs (table 4) was the lowest variability 

 found for any organ at any age. The corresponding coefficient 

 of variation for total body weight was 14.8. At six weeks, the 

 coefficient of variation for the brain, males only, was 12.2, the 

 coefficient for the corresponding total body weights being 36.2. 



