WEIGHTS OF ORGANS IN UNDERFED YOUNG RATS 149 



there appears to be a distinct tendency to an increase in the 

 weight of the hypophysis in rats held at constant weight from 

 three to ten weeks of age. In absolute weight, this corresponds 

 in the male to an increase from 0.00178 gram to 0.0021 gram, 

 an increase of about 18 per cent. In the female, the corre- 

 sponding increase is from 0.00168 gram (0.00178 gram, less cor- 

 rection for difference in body-weight) to 0.0020 gram, an increase 

 of 19 per cent. 



As already shown, the suprarenal glands differ from the hy- 

 pophysis in that they undergo a marked sexual diff erentation in 

 weight during their growth while the body-weight is held con- 

 stant. The impulse to sexual differentiation in these glands 

 therefore appears stronger in the suprarenals than in the hy- 

 pophysis. Final conclusions should be guarded, however, until 

 a larger number of observations is available. 



In the adult rat during inanition the weight of the hypophysis 

 decreases in nearly the same proportion as the body-weight 

 (Jackson '15 c). 



DISCUSSION 



With reference to their growth tendency in young rats held 

 at constant body-weight, the organs may be divided into three 

 classes: (1) those in which the growth tendency is so strong that 

 they continue to increase, even when the body-weight is held 

 constant; (2) those which approximately hold their weight con- 

 stant under these conditions; and (3) those which are unable 

 to maintain themselves and lose in weight. 



According to this scheme, the organs in rats held at constant 

 body-weight from the age of three to that of ten weeks are 

 grouped in table 25. No grouping of this sort can be entirely 

 satisfactory, because in some cases organs are intermediate in 

 position, and especially because (as has been shown) in many 

 cases the weight of an organ will vary according to the age at 

 which the experiment was begun and the length of the period. 

 For example, the liver, which in the three-to-ten weeks experi- 

 ment shows but a slight gain, shows a larger gain at earlier times, 

 and a loss at later and longer periods. 



