HEREDITY OF HAIR-LENGTH IN GUINEA-PIGS. 



2. CHARACTER OF FOLLICLE ACTIVITY IN THE PRODUCTION OF SHORT 



AND OF LONG HAIR. 



If an examination is made of the longest hairs plucked from the back 

 of an adult short-haired guinea-pig, it is found that the hairs narrow toward 

 the base, owing to a diminution in the diameter of the medulla, which is 

 entirely wanting in the follicle (see Fig. I, A). Such a hair has ceased to 



grow, having completed a definite growth cycle, 

 and will sooner or later be shed. This growth 

 cycle was nearing its end when the follicle ceased 

 to form medullary substance. In ordinary 

 guinea-pigs the hair stops growing when it has 

 reached a length of about 4 cm. In long-haired 

 guinea-pigs the hair-follicle does not cease form- 

 ing medullary substance when a definite hair- 

 length has been attained. The growth of the 

 hair is indefinite, ending only with the degenera- 

 tion of the follicle itself. The time when this 

 occurs is determined to some extent by the physi- 

 cal condition of the animal. The lifetime of a 

 hair-follicle of this sort may be prolonged by 

 good care, as fanciers well know. Pregnant or 

 nursing mother guinea-pigs frequently lose their 

 longest hair. Insufficient or improper food is 

 likely to have a similar result in either sex. Ac- 

 cordingly, the fancier gives careful attention to 

 the diet of long-haired animals intended for ex- 

 hibition, and often protects the hair in special 

 ways from mechanical injury. There is, how- 

 ever, no reason to suppose that such care induces 

 indefinite activity of the hair-follicle. On the 

 contrary, animals of a short-haired race, under 

 the best of care, will form only hairs of deter- 

 minate length, whereas animals of a long-haired 

 race, however much abused, will, so long as they 

 live, continue to form hairs of indeterminate 

 growth. The two types of hair-growth are quite distinct, and are sharply 

 alternative in heredity. They are probably paralleled in our own head- and 

 body-hairs respectively, the former being of indeterminate, the latter of 

 determinate growth. 



FIG. i. A, Base of hair, fully 

 grown, of determinate growth type. 



B , Base of hair of indeterminate 

 growth. From camera drawings, same 

 magnification. 



C, Cortex ; F, follicle ; M, me- 

 dulla. 



