POSTNATAL GROWTH IN THE ALBINO RAT 43 



show this to be true also for other organs. Some difference 

 between man and rat might naturally be expected, however, 

 since the rat is born in a more immature condition. 



The variability in the weight of the body must also depend 

 ultimately upon the variability of its component parts. As to 

 the variability of the component parts of the rat, we have data 

 for the head and viscera only, A brief summary of the coeffi- 

 cients of variation in round numbers (from tables 2 to 15; sexes 

 together, excepting gonads and brain) is given in table I. 



The difference in variation between the sexes is usually not 

 very marked (as may be noted in tables 2 to 15) ; although there 

 appears to be a tendency to greater variability in the male, both 

 in body weight and in weight of the indi^ddual viscera. In the 

 table above it may be noted that the head and head organs 

 (brain, eyeballs) form a group'' of small variability (average coeffi- 

 cient 10 to 12) which is usually far below that for the body as a 

 whole (average 19). The other organs are more variable than 

 the body. The lungs, kidneys, heart" liver, and suprarenals form 

 a moderately variable group (average 21 to 26) while the gonads, 

 thymus, spleen and intestinal canal (with contents) are exceed- 

 ingly variable (average 29 to 43) . 



The average coefficient of variation for the viscera examined 

 is 25. It is noteworthy, however, that the average coefficient of 

 variation is lowest at birth and one week, and highest at three 

 weeks. This agrees with what is found also for the body as a 

 whole. 



A brief summary of the coefficients of correlation between the 

 body weight and the weight of the individual viscera (from tables 

 3 to 15, sexes together, excepting brain and gonads) is given in 

 round numbers in table J. 



In this table likewise the differences according to sex are dis- 

 regarded, but even when these are taken into account (cf. tables 

 3 to 15) the general relations are not materially changed. In 

 about two-thirds of the cases, however, the coefficient of correla- 

 tion is higher in the males than in the corresponding females; 



^ The blood, the percentage weight of which in adult rats, according to Chisolm 

 ('11), has a coefficient of variation of 10.7, possibly also belongs in this group. 



