118 SHINKISHI HATAT 



We found by comparing our data with those obtained by Dr. 

 Jackson from his rats that our rats had a constantly, though 

 slightly, smaller heart and kidneys, but a heavier liver. Just 

 what the cause of these differences may have been was not 

 determined. 



3. Influence of age and season. A third cause of disturbance 

 may be due to the age and season. As has been already stated, 

 I have not taken either age or season into account, but have 

 purposely taken the rats at random in order that my data may 

 meet the usual conditions. However, there can be little ques- 

 tion that age and season influence the weight of some of the 

 organs at least. For instance, the weight of the heart is slightly 

 larger in older than in younger rats of the same body weight. 

 The weight of the sexual glands also may vary slightly according 

 to the season. Recent w^ork by Seidell and Fenger ('13) demon- 

 strates a seasonal variation of the iodin content of the thyroid 

 gland. 



4-. Sex differences. The sex differences are so marked in the 

 case of some of the organs that different formulas were needed 

 to represent the two sexes. However, in the majority of in- 

 stances, we failed to distinguish any difference according to sex, 

 so far as weight relations are concerned, and consequently only 

 one formula for each organ, namely, that based on the male 

 data, was worked out. It may however so happen that certain 

 organs react differently according to sex under similar condi- 

 tions. For instance, in the albino rat removal of the sex glands 

 from the male produces an enlargement of the hypophysis, while 

 in the spayed female, the hypophysis is not affected (Hatai). 



Doubtless a more precise analysis of the environmental con- 

 ditions would enable us to extend the list of factors modifying 

 the growth of the organs, but those which have just been enu- 

 merated are evidently worth consideration and the control of 

 them will doubtless go far to reduce the variability of the data. 



