26 C. M. JACKSON 



They were also found relatively much larger in wild rats, due 

 probably to difference in exercise and diet. 



5. Thymus 



On account of inadequate data, Dr. Hatai was unable to con- 

 struct a satisfactory fonnula for the growth in absolute weight 

 of the thymus in the albino rat. 



In my own data grouped according to age periods (table 6, 

 fig. 2 b), it appears that the thymus increases, in the male, from 

 about 0.15 per cent of the body weight at birth to 0.24 per cent 

 at seven days, and to a maximum of 0.38 per cent at twenty 

 days. Thereafter it decreases, and at one year forms an average 

 of only 0.02 per cent of the body. The relative weight under- 

 goes a similar change in the female with no significant difference 

 according to sex. 



A unique feature of the thymus is its decrease in absolute as 

 well as in relative weight. As shown in table 6, the average 

 absolute weight at five months is slightly smaller than at ten 

 weeks, and at one year it has undergone a very striking decrease. 

 This is of course in connection with the process of involution, 

 following the age of puberty, whereby the thymus is largely 

 transformed into a mass of adipose tissue. 



As would necessarily follow from the decrease in the absolute 

 weight of the thymus during the process of involution, there is 

 actually a negative correlation at five months between the thymus 

 and body weights. During the earlier life, on the other hand, 

 there is a well marked positive correlation, the coefficient increas- 

 ing from 0.668 at birth to 0.904 at six weeks (the high figures 

 being partly due to 'spurioUs correlation'). 



The coefficient of variation in absolute weight is high, increas- 

 ing from 30.9 at birth to 50.4 at six weeks. Later, however, 

 when a greater variation might naturally be expected during the 

 involution process, it is actually less, the coefficient being 25.3 

 and 22.2. For the percentage weight, the coefficient of variation, 

 though still high, is considerably smaller than that for the abso- 

 lute weight, excepting the periods at ten weeks and five months. 



