POSTNATAL GROWTH IN THE ALBINO RAT 13 



this indication, however, as it must be remembered that the 

 nmnber of observations is comparatively small, and the differ- 

 ences are usually within the limits of probable error. It is note- 

 worthy, however, that Hatai ('08) likewise found for the body 

 weight and for various skull measurements of the albino rat a 

 tendency to greater variability in the male. 



At the ages of twenty days and six weeks, the standard devi- 

 ation and coefficient of variation were also calculated for the 

 corresponding gross body weights. At twenty days, the coeffi- 

 cient of variation was, for the males 28.7 (as compared with 

 28.4 for the net body), and for the female 18.4 (17.5 for the net) ; 

 at six weeks, for the male 17.7 (17.4 for the net), and for the 

 female 24.6 (24.7 for the net). It is therefore evident that there 

 is no material difference between the coefficients of variation for 

 the gross and for the corresponding net body weights. 



For a larger series of 56 males and 36 females newborn, cor- 

 responding only in part to those given for net body weight, the 

 coefficient of variation for the gross body weight of the male was 

 14.0 (13.6 for the net), and of the feniale 9.2 (9.9 for the net). 

 A larger series was also calculated for the gross body weight at 

 twenty days and six weeks, including all those in table 1. In 

 this larger series (which was about twice as large as the smaller) 

 the coefficient of variation at twenty days for the male was 

 24.4 (28.7 in the smaller series) and for the female, 29.4 (18.4 

 in the smaller series). At six weeks, the coefficient of variation 

 in the larger series for the male was 20.8 (17.7 in the smaller 

 series) and for the female, 24.2 (24.6 in the smaller series). This 

 would seem to indicate that the great difference in the coefficient 

 of variation between the sexes of the smaller series at twenty 

 days and at six weeks is due to an abnormally small variation 

 among the females at twenty days and among the males at six 

 weeks. This should be remembered in considering the varia- 

 bility of the organs, which correspond to the smaller series and 

 are, as will be shown later, all more or less closely correlated with 

 the body weight. The necessity for caution in drawing con- 

 clusions as to variability from a comparatively small number of 

 observations is thus emphasized. 



