THE GROWTH OF EPIDERMAL STRUCTURES 



151 



same is true of birds' feathers. Since much is known about hair patterns 

 and the factors controlling them, we shall consider them in terms of the 

 theory developed above. 



Three different types of follicular activity may be distinguished : 

 (a) Periodic activity with neighbouring follicles not in phase, i.e. each 

 follicle behaves independently with periods of growth followed by periods 

 of rest. This may be referred to as " mosaic growth " and probably 

 represents the basic pattern of isolated follicular activity. 



Fig. 62. The growth waves on rat skin demonstrated by the dye- 

 absorption of growing follicles. Regeneration of hair at 3, 8, 30 and 35 

 days after shearing the area of A, B, C, D. Similarly-hatched areas are 

 those in which the hair is growing at the same time and in which the 

 alloxazine pigmentation would develop in response to administration of 

 the compound at about that time. (Reproduced with the kind permission 

 of Professor Haddow and the Editor of Nature). 



(b) Continuous activity as found in sheep and on the human head. This 

 is a special case of (a) with exceptionally long growing phases and a short 

 resting phase. 



(c) Periodic activity with neighbouring follicles in phase as is found in 

 rats and mice (Butcher, 1934; Haddow et al., 1945; Fraser and Nay, 

 1953). At any time most of the follicles are quiescent and activity is 

 confined to small areas which may form recognizable wave-fronts (Fig. 62). 



Intensive studies have been made of hair growth in man, sheep, some 

 rodents and fur-bearing animals. Quasi-continuous activity (b) is rare: 



