252 SHINKI3HI HATAI 



and 28 Wistar). The animals were of the usual size and weight 

 and were in good condition. 



As to the method of dissection of the thymus, it is important 

 to remember only that the large lymphatic glands, as well as 

 the fat, which lie close to it, are not to be included in its weight. 



The observed weights of the thymus gland for various ages, 

 as obtained from this material, are given in table 1. 



Using the observed weights, as given in table 1, formulas have 

 been devised by which the change in the weight according to 

 age may be expressed'. Formula 1 applies to rats less than 95 

 days of age and is as follows : 



Thymus = 0.01XlO' '{''''* + ''''' (T5 -)-'■'''' i^^-'y] (1) 



Formula 2 applies to rats 95 days or more of age and is as 

 follows : 



Thymus = 0.3903 - 0.00139 (age) + 0.00000128 (age)^ (2) 



Using these formulas we obtain the computed weights of the 

 thymus according to age as given in table 2, and from these the 

 graph shown in chart 1 has been constructed. 



As will be seen from the chart, the thymus gland increases in 

 weight very rapidly from birth up to about 85 days in age at 

 which time the gland reaches its maximum. This rapid increase 

 in weight is followed in turn by a steady decrease for the rest of 

 life — the involutionary process. The period of the maximum 

 weight corresponds nearly with the end of the rapid growth of 

 the sex glands in weight, as will be seen from the following 

 relations: 



The body weight of the ordinary albino male rat at 85 days of 

 age is 144 grams (Donaldson '06). The corresponding weight 

 of the testes would be about 1.76 grams (Hatai '13). If we take 

 a body weight of 400 grams as the approximate maximum for 

 a fully grown male rat under usual laboratory conditions, the 

 corresponding weight of the testes would be 3.06 grams. From 

 these data we find that nearly 57 per cent of the final weight 

 has been attained by the testes when the rat is 85 days old. 



