110 CM. JACKSON 



In order to show more clearly this change in the ratio of tail- 

 length to body-length, the individual ratios corresponding to 

 the 450 observations on rats from all sources (including the 148 

 Wistar rats) were plotted according to body-weight in figure 2. 

 As the sexes are distinguished in the entries, it is evident that 

 the females tend to have a higher tail-ratio. The curve has 

 been drawn through the averages at various periods (sexes 

 combined). It is smoothed free hand, as the labor of con- 

 structing the curve more accurately by mathematical methods 

 did not seem justified. 



Special attention is called to one apparent discrepancy between 

 the curve in figure 2 and the data in table 3 a. In the latter, it 

 appears that the tail reaches at six weeks an apparent maximum 

 ratio of 0.88 (sexes combined), which is maintained nearly con- 

 stant at succeeding periods. In the chart, however, it is seen 

 that in rats above 300 grams the average ratio drops to nearl}^ 

 0.80. These heavy rats are nearly all from the Wistar series, 

 and are all males. It is therefore evident that the drop in the 

 tail-ratio curve is in part due to the fact that no females are 

 included in the higher body-weights. Even taking this into 

 account, however, there is still a decrease in the tail-ratio of 

 the male from a maximum of about 0.86 or 0.87 in rats between 

 50 and 200 grams body-weight, to nearly 0.80 in rats above 

 250 grams. (In 7 male rats between 350 and 400 grams, not 

 shown in figure 2, the ratios were slightly above the 0.80 line, 

 the average being 0.81).^ 



" Since the completion of the present paper, I have received, through the court- 

 esy of Professor Donaldson, a manuscript copy of reference tables compiled at 

 The Wistar Institute by formulas for various measurements of the albino rat. 

 These include the body-lengths and tail-lengths, by sexes, from newborn to adult. 

 From these data I have calculated the tail-ratios and find the result in general 

 agreement with the curve shown in figure 2. The tail-ratios calculated from the 

 Wistar tables are somewhat lower, corresponding to body-weights from 30 to 

 100 grams, however. They also increase steadily, so that at body-weights above 

 200 grams they lie slightly above the curve in figure 2. The tail-ratio according 

 to the Wistar tables is about 4 per cent higher in the female throughout, when 

 the sexes of equal body-length or body-weight are compared. In rats above 300 

 grams body-weight, the tail-ratio is about 0.S6 in the male and 0.89 in the female. 



