56 HELEN DEAN KING 







reduces the size of the guard hairs with the exception of a 

 few around the head, and causes a bend or twist in all hairs 

 of the body including the vibrissae. Because of the modifica- 

 tion of the vibrissae, this mutation can be detected in very 

 young rats. The characteristic curl of the hair becomes very 

 pronounced when the rats are 2 weeks old (fig. 21), and the 

 vibrissae are then spiral or bent inward and hooked at the 

 ends (fig. 22). When the rats are about 25 days old the curl 

 begins to disappear and soon the coat looks much like that in 

 normal rats, although it has a soft downy appearance be- 

 cause of the absence of guard hairs (fig. 23). This change in 

 the coat is due probably, as Feldman ('35) has suggested, to 

 the fact that the vigorous pelages of young adults are able to 

 overcome in a large measure the destructive effects of the gene. 



The curl reappears in the hair of the mid-dorsal surface 

 when the mutants are about 7 months old (fig. 24), and in the 

 course of 2 or 3 months has extended over the entire coat. 

 This mutation shows its most pronounced effects in old rats 

 where the entire dorsal surface is covered with very curly 

 hair (fig. 25). On the ventral the longer hairs tend to break 

 off or to disappear in various places, thus showing patches of 

 skin between ridges of curly hair (fig. 26). 



A histological examination of sections of the skin of curly 

 mutants of various ages disclosed no apparent abnormality 

 in the hair follicles. The wavy appearance of the coat seems 

 to be due to the fact that at intervals along the shaft the 

 cortical layer of the hair becomes very thin, and a bend or 

 a twist occurs at these points. 



This mutation was first. discovered in a male of the seven- 

 teenth generation, born when the mother was 6 months old. 

 Early changes in the pelage had not been noted, but the un- 

 usual appearance of the coat when the curl had returned after 

 the 'latent' period aroused suspicion that a new mutant type 

 had appeared. Fortunately, this male had sired a litter which 

 had been saved for study. The four females in this litter had 

 normal coats; the two males were curly. Nine litters were 

 obtained from the matings of these curly males with five nor- 

 mal gray females, two of which were their sisters. These 



