MOULT AND REGENERATION OF PELAGE IN MICE 77 



Frequent examinations, somewhat more at random, were also 

 made upon the general stock, including the subspecies sonoriensis 

 (Le Conte)and rubidus Osgood, as well as a few specimens of 

 P. eremicus fraterculus (Miller) and P. calif ornicus insignis Rhoads. 

 By etherizing the animals and parting the fur, it was possible to 

 follow the moult from the time of the first appearance of the new 

 hairs through the skin. 



At birth the body is devoid of hair and pigment except for the 

 vibrissae and supraorbital cilia. On the second day the upper 

 parts begin to assume a bluish-black coler and the hair may be 

 seen coming through the skin of the pigmented area. A day or 

 two later, the ventral white hair may be observed. 



At the age of four to five weeks, the young are, as a rule, in full 

 Juvenal pelage. There are no further traces of pigment in the 

 skin, which is now flesh color. This pelage, like the later ones, 

 is made up of a fine soft underfur and a thinner coat of much 

 longer and coarser overhair. As is the case in the adult pelages 

 of many other rodents, the hairs of the underfur are banded or 

 ticked (agouti), being of a blackish plumbeous or slate color 

 basally, with a narrow subterminal zone of pallid mouse gray, 

 while the tips are black.^ The overhairs are not of the agouti 

 type, lacking the subterminal band. The general effect on the 

 dorsal surface may be described as between neutral and deep 

 neutral gray. 



The Juvenal pelage of the ventral surface, like that of the dor- 

 sum, is made up of underfur and overhairs. Basally, the color 

 is the same as in that of the dorsal surface, but the distal region 

 is white. The lateral line of demarcation between the dorsal 

 and ventral surfaces is very sharply defined (fi.g. 5) . 



The microscopic structure of the hairs in the juvenal pelage is 

 essentially the same as described by Sumner ('18) for the adult. 

 There is, however, a very evident difference in the proportionate 

 number of the different kinds of hairs. The slender hairs with 

 but a single axial row of pigment bodies, alternating with the air 

 spaces, are much more numerous, while the yellow pigment is 

 much reduced in the subterminal bands. The overhairs are 



^ Color descriptions are based on Ridgway's key. 



