POSTNATAL GROWTH IN THE ALBINO RAT 37 



respondingly low, 0.347 to 0.583 (except at seven days) for the 

 sexes combined. In the sexes, separately, however, the correla- 

 tion is usually somewhat higher, as might be expected, since the 

 sexes show a characteristic difference in the relative weight of 

 the suprarenals. 



12. Kidneys 



The relative (percentage) weight of the kidneys, reckoned 

 according to Hatai's formula, is shown by the curve in figure 6 b. 

 The kidneys appear in the newborn male to form 0.90 per cent 

 of the body weight at 5 grams, which increases to a maximum 

 of 1.53 per cent at 10 grams, thereafter decreasing to 1.38 per 

 cent at 20 grams, 1.10 per cent at 50 grams, 0.91 per cent at 130 

 grams, 0.85 per cent at 200 grams, 0.82 per cent at 300 grams, 

 and 0.80 per cent at 400 grams. 



When grouped according to age periods, my own data (table 

 14, fig. 6 b) show the relative (percentage) weight of the kidneys 

 to increase in the male from 0.96 per cent of the body weight 

 at birth to 1.29 per cent at seven days and to a maximum of 

 1.44 per cent at twenty days. Thereafter it decreases to 1.27 

 per cent at six weeks, 1.03 per cent at ten weeks, 0.93 per cent 

 at five months, and 0.95 per cent at one year. In the females, 

 the average percentage weight of the kidneys is usually slightly 

 higher than in the male of the same age, but nearly the same as 

 that of a male, with the same body weight. The agreement of 

 my data with the curve derived from Hatai's formula is remark- 

 ably close (fig. 6 b) . 



The coefficient of variation of the kidneys in absolute weight 

 (table 14) is greater than that of the body in the earlier periods, 

 reaching a maximum of 33.7 at twenty days. Later, however, 

 the kidneys become less variable than the body weight, the coef- 

 ficient being 15 to 19. The variation in the percentage weight 

 is low, especially from seven days onward. The coefficient of 

 correlation between kidneys and body weight is correspondingly 

 high, being 0.703. at birth, and 0.788 to 0.955 at later age periods, 

 (partly due to 'spurious correlation'). No constant difference in 

 variation is noticeable according to age or sex. 



