THE KERATINIZATION PROCESS 215 



constriction to the fully stabilized level may be divided into two distinct 

 parts : 



(a) A lower zone (Fig. 90, zone D) in which the synthesis and orientation 

 of the fibrous protein itself seems largely complete, but in which the 

 structure is poorly stabilized. 



(b) An upper zone (Fig. 90, zone E) in which stability rapidly develops. 



(b) 



Fig. 92. Histochemical methods of demonstrating the unconsolidated 

 zone D of the pre-keratin. Birefringent regions are shown in black. 



(a) Destruction of orientation in zone D by heating for 30 sec at 90° C. 



(b) Result of tryptic digestion of a follicle for some hours. D is removed, 

 the inner root sheath is resistant above the level B and the earlier harden- 

 ing of the fibre cuticle is apparent, (c) Destruction of DR due to the 

 penetration of dilute acid, (d) Result of treatment with dilute alkali 

 which acts destructively even on the partly-hardened regions, zone E. 



From Mercer (1949b). 



These zones are well distinguished by the tests illustrated in Fig. 92. 

 Briefly, many reagents and treatments dissolve only the lower zone (D, 

 Fig. 90) of weaker stability; more violent treatment is required to derange 

 the subsequent levels (Fig. 92d). The exact range of these levels is not the 

 same in all types of follicle, but Hardy's work on the mouse follicle shows 

 that the sequence of changes is the same. 



In experiments on plucked hair roots the present writer (1949b) showed 

 that the cortical orientation (observed as double refraction) was destroyed 



