84 H. H. COLLINS 



head precedes replacement on the hips and hind Hmbs — an in- 

 version of the natural order. Growth is almost simultaneous on 

 the two denuded areas of the ventral surface, while the pelage 

 of the hips was last to be replaced. 



It will be noted also that the mode of replacement on the head 

 was radically different from the normal process. Typically, 

 growth proceeds, a) dorsally, from the tip of the snout to the 

 anterior insertions of the ears, or, in some cases, posteriorly to a 

 point midway between the ears; h) from the shoulders, anteriorly 

 over the back of the neck to the ears. In contrast to this con- 

 dition, in regeneration the new hair appears almost simultane- 

 ously over the whole of the depilated area. 



It is but natural to suppose that the normal process of growth 

 would be less profoundly modified were regeneration to occur 

 immediately before or during the normal moult. But this does 

 not seem to be the case. In mice operated upon at the age of 

 six weeks, with the normal moult well under way, regeneration 

 occurred in essentially the same manner, as regards sequences and 

 directions of growth. Even in an extreme case, in which the 

 entire body except the head and rump were covered with post- 

 juvenal pelage, replacement on the head was somewhat in 

 advance of that on the rump. Replacement on the head w^as 

 fairly uniform, though the snout below the eyes was last to be 

 invested — an inversion of the normal condition. 



In exceptional cases the normal order is not so completely dis- 

 guised, as shown by the history of another brood (that of 9 105), 

 the four members of which were operated upon in the same 

 manner and at the same age as the former brood. 



The first suggestion of the normal process, such as would 

 occur without operation, appeared on the depilated area of the 

 throat about a week after the hair was removed. Here there 

 was a perceptible darkening at the point of the jaw and along the 

 anterior face of the forelimb, thus outlining the region where the 

 post Juvenal pelage normally first appears. At the same time the 

 rest of the exposed area showed no signs of pigment formation. 

 Within a few days incoming pelage appeared on this pigmented 

 area, the rest of the region being covered some days later. Typi- 



