284 /. /. Christian 



animals, probably including the dominant animals, were less affected by 

 increased density than those in any other weight group. 



It was mentioned earlier than in response to stimuli which evoke in- 

 creased adrenocortical activity there is often inhibition of growth resulting 

 either from direct inhibition of growth by carbohydrate-active corticoids, 

 by inhibition of the secretion of pituitary growth hormone, or both. Evi- 

 dence has been presented indicating that there is inhibition of growth with 

 increased population density. Therefore, one might question the comparison 

 of organ weight data from mice from populations of high density with those 

 from segregated mice, as mice from the high-density populations may be 

 older than segregated controls for the same body weight. However, if it 

 were possible to correct for age, the differences with respect to body weight 

 between high-density populations and segregated controls would be even 

 greater. Therefore, comparisons with respect to body weight will err on the 

 conservative side. 



The zonae fasciculatae of female mice of all sizes from high populations 

 were appreciably wider than their controls as a result of cellular hyper- 

 plasia and hypertrophy, although the presumptive increase in adrenocorti- 

 cal secretion was not indicated by the thymus weights. If anything, the 

 thymuses of females from the experimental populations were heavier than 

 those of the segregated controls, possibly a reflection of a diminished secre- 

 tion of the sex steroids. 



On the other hand the preputial glands respond to stimulation by ACTH 

 (Jacot and Selye, 1951; Hess et al, 1952, 1953; Rennels et al., 1953), and 

 the preputials of female mice from high-density populations were heavier 

 than those from their segregated controls. Therefore, it is possible that the 

 increase in the weights of the preputial glands of these mice may have re- 

 sulted from an increased secretion of ACTH. This problem will be discussed 

 in more detail subsequently. 



The effect of increased population density on adrenocortical function of 

 voles {Microtus pennsylv aniens) in freely growing confined populations has 

 also been studied. The experimental procedure used for these populations 

 was essentially the same as for the house mice except that eosinophil 

 coimts were used to measure adrenocortical function in the three popula- 

 tions of voles (Louch, 1956). The use of eosinophil counts has an obvious 

 advantage over adrenal weight for assessing adrenocortical function in 

 that the animals do not have to be sacrificed, especially when various factors 

 contributing to variation in the counts are taken into consideration ( Louch 

 et al., 1953) . In such long-term studies as this with repeated counts, there 

 can be little doubt that the eosinophil counts reflect adrenocortical function 

 rather than medullary. There was a significant negative correlation between 



