78 H. H. COLLINS 



attenuated at the base, being no larger at this level than the hairs 

 of the underfur. Both kinds of hairs are very much flattened, 

 but the larger ones show no local attenuations such as are de- 

 scribed by Barrett-Hamilton ('16) for Mus musculus, though 

 this appearance may be simulated by torsion. 



In microscopic structure, the vibrissas are markedly different 

 from the hairs just mentioned. Though these are the largest 

 hairs on the body, there is but one axial row of lacunae containing 

 a relatively small amount of pigment. Most of the pigment 

 occurs in the cortex as. small granules arranged in longitudinal 

 striae. This cortical pigment extends to the base, but gradually 

 disappears toward the tip. This is the reverse of the arrange- 

 ment in the body hairs, in which the greater part of the pigment 

 is found in the axial region arranged in from two to four rows of 

 lacunae in all except the smallest hairs. Furthermore, in the 

 bod}^ hairs, the cortical pigment, which is restricted mainly to the 

 superficial region of the cortex, is most dense in the terminal zone, 

 gradually disappearing toward the middle region. The lower 

 vibrissae are devoid of pigment almost or quite to the base, but 

 this terminal white region becomes much reduced dorsally. The 

 structure of the two supraorbital cilia and of the hairs of the tail 

 is similar to that of the vibrissae. In certain pelage removal 

 experiments to be described later, it will be noted that vibrissae 

 and body hairs are not regenerated in the same manner. This 

 fact suggests the possibility of some sort of correlation between 

 the morphological and physiological differences. 



The transition from the juvenal to the postjuvenal pelage 

 usuall}^ begins at the age of six weeks and is completed about 

 eight weeks later. The new pelage first appears on the throat 

 near the angle of the jaw, or rarely on the. anterior surface of the 

 forelimb along the lateral line.- Growth proceeds toward the 

 median ventral line of the head and, at the same time, anteriorly 

 under the eye and ear and posteriorly over the forelimb and 

 shoulder. From these regions, it passes posteriorly above the 

 ventral white to the hind limbs, at the same time creeping up 

 toward the dorsal median line (figs. 1 to 3). 



- The line of demarcation between the white hair of the ventral surface and 

 the dark hair of the dorsum. 



