THE GROWTH OT EPIDERMAL STRUCTURES 



135 



production and keratin production, are under the control of the sex 

 hormones (p. 144). The effects of various hormones on hair growth have 

 been described by Mohn (1958) and on feather by Lillie (1942). 



The most obvious effect of the sex hormones is on hair and feathers. 

 The action of the male hormone appears to affect directly the length of the 

 growth period of certain follicles so that longer and stouter hairs (or 



Table 6. Some Renewal Times for Epidermal Tissues. 



* Taken from Leblond and Storey (1951 and 1956). 



t From Piatt (1960), other figures will be found in Hooper (1956), 

 Price (1958), Meyer et al. (1960) and Scheving (1959). 



feathers) are produced, although the type may also be affected. That other 

 quite extraneous substances may act as stimulants is shown, for example, 

 by the marked effect of scarlet fever toxin (Heyningen, 1950). 



Mitosis in the basal layer 



Although very little that is not hypothetical can be said about the control 

 of the overall patterns of growth, there is better experimental evidence 

 concerning the mechanisms of short period fluctuations. 



While the average rate of cell replacement in the epidermis is relatively 

 steady, Bullough has established the presence of diurnal cycles in the 



