2. Endocrines and Populations 249 



largely of metabolites of steroids. Furthermore, a large proportion of the 

 steroids are excreted in the feces in some species (Barry et al., 1952; Brad- 

 low et al., 1954) . Finally, techniques for the measurement of urinary steroids 

 have not been developed for use in the field. It should be apparent from 

 the foregoing discussion that assessing adrenocortical activity by physio- 

 logic or biochemical means in living wild animals or animals which have 

 been live-trapped is fraught with difficulty, and under most circumstances 

 measurements as such of the blood steroids will probably reflect the im- 

 mediate situation of the animal. In general, weight and histologic criteria 

 from animals killed suddenly seem to offer the most dependable informa- 

 tion at the present time for studying population phenomena, although the 

 other procedure should be explored further. Even though there is always a 

 question of the presumptive relationship between hypertrophy, morpho- 

 logic change, and function, in most circumstances it is generally accepted 

 that cortical hypertrophy reflects functional change. The probable depar- 

 tures from this general statement have been discussed above. 



/. Thymicolymphatic system. In the foregoing account it has been stated 

 that the carbohydrate-active corticoids produce involution of the thymico- 

 lymphatic system, mainly by effecting involution of the lymphoid elements 

 proper. Therefore the weights of the thymus, lymph nodes, and spleen 

 provide useful indicators of increased adrenocortical activity, especially if 

 appropriate histologic checks are used. 



g. Thymus weight. Cortisone, hydrocortisone, and endogenous adrenal 

 corticoids cause thymic involution (Dougherty, 1952a, b; Weaver, 1955). 

 Increased phagocytosis, edema of the connective tissue stroma, and hyper- 

 plasia of the reticulum are associated with the destruction of the lymphoid 

 tissue proper (Gordon, 1955). If the reaction is severe enough, only the 

 stroma and a modified reticulum may remain, with no distinction between 

 the cortex and medulla of thymus and lymph nodes in degree of involution 

 (Weaver, 1955) . The weight of the thymus therefore will be greatly de- 

 decreased. Within a few hours following stimulation there is a marked 

 edema, and during this period of edema there is a marked reduction in the 

 number of lymphocytes; those remaining exhibit degenerative changes 

 (Dougherty, 1952b). The effect is greatest on the small and medium-sized 

 lymphocytes and thymocytes. These exhibit pycnosis, karyolysis, frag- 

 mentation, and other degenerative changes. The fragments arc phagocy- 

 tized by macrophages in reaction centers of the lymph nodes and are 

 carried off in the lymphatics. Mitosis ceases in the lymphoid organs with 

 treatment with cortisone, hydrocortisone, or with chronic stimulation of 

 the adrenal cortices by endogenous or exogenous ACTH (Weaver, 1955; 

 Gordon, 1955) . These changes are common to the lymphoid follicles where- 

 ever they occur — spleen, nodes, gastrointestinal tract — but are most 



