154 KERATIN AND KERATINIZATION 



apart) to share the same diffusion fields. It is in such conditions that 

 growth is transmitted in a wave-form. This view is essentially that of 

 Durward and Rudall if we equate their " resting stage inertia " with 

 " accumulation of inhibitor." In these terms the development of growth 



W 



Ht \\\\\\\\\\\ 



m 



CROWING FOLLICLE 



(b) 



Fig. 64. (a) The generation of a growth wave in a population of follicles 

 whose growth is accompanied by the build-up of inhibitor molecules 

 (small dots) which on reaching a critical concentration cause growth to 

 regress. In the top line growth is advancing towards the right as follicles 

 recommence growth. After growth has persisted for sufficient time, 

 sufficient inhibitor accumulates to cause the follicle to cease growth 

 (bottom line). Growing follicles are indicated by a line with an enlarge- 

 ment at the end, and non-growing follicles as a shorter line. 



(b) The lower illustration is an indication of how a typical growth wave, 

 commencing on the ventral surface of an animal, can travel dorsally. 



waves can be explained as follows. We consider a population of resting 

 follicles and suppose that at A the concentration of inhibitor falls below 

 the threshold value permitting growth to recommence (Fig. 64). In 

 response to the demands of the growing cells, which initially include cells 

 of the adjacent epidermis, and outer root sheath, the vascularity beneath A 

 increases. If now the concentration of inhibitor in the neighbour of 



