POSTNATAL GROWTH IN THE ALBINO RAT 23 



For the same data, the coefficient of correlation between brain 

 weight and (gross) body weights was, at birth, male, 0.736; 

 female, 0.566, total, 0.690 ±0.037; at three weeks, male only, 

 0.783 ± 0.049 ; at six weeks, male only, 0.883 ± 0.020. These high 

 figures are due in part to 'spurious correlation' (as will be explained 

 later). In general, Donaldson ('08) found (in 680 records at 

 various ages) the coefficient of correlation between brain weight 

 and body weight to be 0.7639± 0.0108. 



Hatai ('08) found for the cranial capacity of adult albino rats a 

 coefficient of variation of about 6.7 in the male, and 7.2 in the 

 female. The coefficient of correlation between cranial capacity 

 and (gross) body weight was 0.516 ±0.074 in the male and 0.692 =t 

 0.058 in the female. 



For the spinal cord, the relative weight (according to Donald- 

 son's formula) increases from about 0.66 per cent of the body 

 at 5 grams to a maximum of 0.77 per cent at 10 to 15 grams, 

 declining to 0.28 per cent at 205 grams and 0.22 per cent at 315 

 grams. From the combined data the coefficient of variation was 

 calculated for the newborn only (48 males, 29 females) and was 

 found to be as follows: male, 17.8 (for total body, 13.1); female 

 12.6 (total body, 9.4) ; total (sexes combined) 16.0 (for total body 

 12.5). The corresponding coefficient of correlation with the body 

 weight was 0.666 ±0.043, which is somewhat lower than that 

 found by Donaldson ('08) for the spinal cord of the rat in general 

 (0.8564 ±0.0071). 



In comparing these results with those for other organs, it must 

 be remembered that the rats from which the brain and spinal 

 cord were derived correspond only in part to those for all the 

 other organs. 



3. Eijeballs 



The relative or percentage weight of the eyeballs for various 

 body weights, when calculated from Hatai's ('13) formula, is 

 shown by the curve in figure 1 b. According to this curve, it 

 is seen that the eyeballs increase in relative weight from about 

 0.59 per cent of the body weight at 5 grams to 0.61 per cent at 

 10 grams, thereafter decreasing steadily, reaching 0.48 per cent 



