PERIODICITY IN HUMAN BEINGS AND MICE 821 



pretation has further presupposed a "basal" level of adrenocortical 

 activity during sleep and/or rest; thus it seems to ignore the intrinsic 

 aspects of 24-hr periodic cortical adrenal secretion and its time rela- 

 tions to other functions which normally also are periodic. 



Figure 13 reveals a rhythm of the gland itself, as evaluated by a 

 morphologic index, the number of mitoses in the cortex. In keeping 

 with earlier data obtained in spot checks at only two times of day on 

 mice (Halberg, Frantz, and Bittner, 1957) and rats (Miihlemann et 



07:30 15:30 23:30 07:30 



Time of Day 



Fig. 13. Mitotic rhythm in adrenal cortical parenchyma and stroma. 



ai, 1955), mitotic activity has reached a peak by the middle of the 

 daily dark period. The relation of mitosis to secretion in the gland has 

 been discussed elsewhere (Halberg, Frantz, and Bittner, 1957) on the 

 basis of the time relations of the daily rhythms in eosinophils and in 

 parenchymal adrenal mitoses: eosinophil counts revealed trends that 

 were roughly opposite to those of cortical mitoses. It was suggested as 

 a "probabilistic" relationship (Rashevsky, 1955) that mitotic activity 

 in the cortex increases daily when the hormone is released from the 

 gland and decreases when hormone secretion is resumed (Fig. 14). 



