108 C. M. JACKSON 



Two facts concerning the relative length of the tail in the 

 normal albino rat are apparent from table 3 a. In the first place, 

 it is evident that at all ages the ratio of the tail-length to the 

 body-length is slightly greater in the female than in the male; 

 that is, the female is relatively long-tailed. By a comparison 

 of the ratios, it appears that the tail of the female on the average 

 is about 4 or 5 per cent longer. 



In the second place, it appears that the ratio of tail-length to 

 bodj^-length increases in the normal rat from an average of about 

 0.36 at birth to 0.48 at one week, 0.66 at three weeks, and 0.88 

 from the age of six weeks upward. That is, the tail becomes 

 progressively relatively longer, being relatively more than twice 

 as long in the adult as at birth. The acceleration of the tail- 

 growth at such a later period may be cited as an instance of 

 the general law of cranio-caudal progression in development 

 (Jackson '09). 



Turning now to the tail-ratio in the rats held at constant 

 body-weight, as shown in table 3 b, and also indicated in the 

 chart in figure 2, it is clear that in rats beginning at three weeks 

 of age there has been a very decided increase in the tail-ratio. 

 The tail at this age evidently continues to elongate, even though 

 the body-weight has been held constant, so that the tail-ratio 

 approaches (although it does not usually quite reach) the normal 

 ratio for rats of corresponding age under normal conditions of 

 growth. 



If we compare the absolute lengths of tail and body in the 

 normal rat at three weeks (table 3 a) with those in rats held at 

 constant body-weight from the age of three weeks to the age 

 of six, eight, ten, thirteen and sixteen weeks, it appears that 

 there has also been a slight increase in the absolute length of 

 the body. The increase in tail-length is considerably greater, 

 however, so the tail-ratio increases as above stated. 



The larger number of rats were held constant from the age of 

 three to the age of ten weeks. Since the average body-weight 

 for the normal series at three weeks is slightly lower than that 

 of the series held constant to ten weeks of age, I have obtained 

 a new normal series of higher body-weight for direct comparison 



