76 H. H. COLLINS 



Passing to the Muridae, it maj- be said that although certain 

 species of this group have long been reared in captivity and used 

 extensively in experimental work, very little is known concerning 

 their changes of pelage. Probably the most complete account is 

 that of Osgood ('09) in his monograph of the genus Peromyscus. 



As stated by Osgood, the mice of this genus pass through three 

 fairly distinct phases due to age — the juvenal (young in first 

 coat), the adolescent, and the adult. According to his descrip- 

 tion of the assumption of the post juvenal or adolescent pelage, 

 ''This [i.e., juvenal] stage is succeeded by the adolescent pelage, 

 which first appears on the middle of the sides. Its growth pro- 

 ceeds rapidly upward on each side until union is effected in the 

 middle of the back, and then incloses the rest of the body, the 

 rump and nape, usually being the last parts to be covered" (p. 20) . 



With reference to seasonal changes, he says: 



The new pelage may be acquired in regular and obvious manner 

 with the fresh coat well distinguished from the old worn one, the growth 

 proceeding from before backward and the middle of the rump being 

 the last part to be invested, or the change may be quite insidious and 

 apparent only upon careful examination. The regular method is fol- 

 lowed in the adults of most species, while the other is more often evident 

 in immature individuals (p. 19). 



My own observations in this field have been, in the main, con- 

 fined to a few species of this same genus. During the past year 

 and a half I have devoted considerable attention to a study of 

 the seasonal and life-cycle changes in the pelage of several races 

 of CaUfornia deer-mice, reared in the murarium of the Scripps 

 Institution. It is the purpose of the present paper to discuss, 

 somew^hat in detail, the normal process of moult, especially with 

 reference to the assumption of the postjuvenal pelage, and, 

 furthermore, to describe certain modifications of this process 

 experimentally induced. 



I take this occasion to express my sincere thanks to Dr. F. B. 

 Sumner for many valuable suggestions and criticisms. 



2. THE POSTJUVENAL MOULT 



The description of this moult is based upon an examination at 

 weekly intervals of a series of twenty specimens of the first cage- 

 born generation of Peromyscus maniculatus garabeU (Baird). 



