THE GROWTH OF EPIDERMAL STRUCTURES 



155 



follicles adjacent to A is also approaching the critical value, and if the 

 follicles are sufficiently close to one another, the increased blood flow will 

 initiate growth in these follicles. By the same process further follicles 

 will be stimulated and growth will travel towards fiasa wave. Behind the 

 front, growth will continue until the inhibitor again accumulates to the 

 critical valve when the follicles will again enter the resting state together. 

 On a simple cylinder (Fig. 64b) (model of the body of an animal) if 

 growth commences along a ventral line, waves will travel dorsally on lines 

 parallel to the initial line as is observed. The extremities, the legs, ears, 



dorsal 



Fig. 65. The result of rotating an area of skin through an angle of 180° 

 and regrafting it as described by Ebling and Johnson (1959). When a 

 growth wave moving dorsally reaches the level AB, hair commences to 

 grow at the dorsal edge of the rotated graft (as if this were not displaced) 

 and travels ventrally. This is contrary to the predictions of a theory based 

 on simple control by accumulated inhibitor. 



etc., pose special conditions which will break the uniformity of the pattern 

 but growth should still proceed on fronts. Inhibitor theory is thus able to 

 describe qualitatively the appearance of waves, but it cannot yet be 

 developed quantitatively. 



Ebling and Johnson (1959) claim to throw doubt on this explanation. 

 They severed areas of rat skin entirely from their dermal connexions 

 and regrafted them after rotation through 180°. On such grafts the 

 growth wave normally proceeding in a ventral-dorsal direction (see Fig. 65) 

 commenced on the dorsal side of the graft and travelled downwards. That 

 is to say the follicles in the graft behaved exactly as they would have if they 

 had not been rotated, whereas the wave transmission theory would predict 

 that the wave on reaching AB would advance dorsalwards across the 

 graft. Ebling and Johnson infer that the follicles are actually independent 

 and the wave advances simply because there is a ventral dorsal gradient 



