Hunt and Wright: Pigmentation in Guinea-pig Hair 179 



the number of the granules observed in 

 sepia hair seems to be practically the 

 same as in black hair. However, since 

 black hair is darker than sepia, it cer- 

 tainly must contain a slightly larger 

 amount of black pigment than sepia 

 hair. 



Red Hair. — Red hair, like black and 

 sepia, has a large amount of granular 

 pigment in the medulla, but unlike 

 them it has very few granules in the 

 cortex. As will appear later, this absence 

 of cortical granules is a significant 

 fact. 



The granules in red hair vary widely 

 in size and shape. Nearly all are 

 spherical, having a diameter, as accur- 

 ately as could be computed, of 0.2 to 0.3 

 micron. It is difficult to see these small 

 ones clearly except when an oil immer- 

 sion objective is used. The largest 

 granules, which are relatively few in 

 number, are elongated. Some of them 

 are as large as 6 micra x 2 micra, about 

 eighteen times as long as the smallest 

 particles. Between these two extremes 

 is found a great variety of intermediate 

 sizes. 



The color of the pigment particles is 

 distinctly yellowish. The difference in 

 color between them and the pigment in 

 black hair is very clearly seen when one 

 compares sections of the two kinds of 

 hair under the same conditions of illumi- 

 nation and magnification. 



Diffuse yellowish pigment is abundant 

 in the cortex of some hairs. Probably 

 it is present in every hair but can be seen 

 only in those sections where it is rela- 

 tively concentrated. It is most dense 

 around the medulla, fading away to 

 invisibility near the surface of the hair. 



Yellow Hair. — The ntimber of gran- 

 ules in the cortex of yellow hair is 

 extremely small. The medulla contains 

 a greater quantity of granular pig- 

 ment, but even this is considerably 

 less than in the medulla of red hair. 

 Comparing red with yellow guinea pigs, 

 one finds that the yellow color is really 

 a dilute red. The difference in the 

 quantity of granular pigment obser\^ed 

 in the hair sections may explain this 

 color dilution. 



The granules in the 3'ellow hair vary 

 miarkedlv in size and color. There are 



a few of the large yellow type such as are 

 found in red hair. Of the smaller 

 granules, a large part seem to be as 

 black as the smaller ones in black hair, 

 while the rest are yellowish. Probably 

 these black granules are more highly 

 oxidized melanin than the yellow ones. 

 Yellow hair is sometimes slightly sooty 

 in appearance. The black granules are 

 undoubtedly responsible for this sooti- 

 ness. 



A slightly yellowish tint in the cortex 

 of a few hairs suggests that diffuse 

 yellow pigment may be present, but is 

 not always visible on account of the 

 thinness of the sections. 



Cream Hair. — The medulla and cortex 

 of cream hair contain a very few gran- 

 ules, apparently black, of approximately 

 the size of the granules in red hair. 

 The sections do not furnish evidence 

 of diffuse pigment, but the dark color 

 and the small ntmiber of the pigment 

 granules can hardly account for the 

 color of the hair, which is not black 

 but cream. Therefore diffuse ^-ellow 

 pigment is probably present, but in such 

 small quantities that in the sections it 

 does not visibly alter the color of trans- 

 mitted light. 



DISCUSSION OF OBSERVATIONS 



The preceding discussion has brought 

 out a nimiber of distinct ways in which 

 red hair differs from black. First, there 

 are differences in the character of the 

 pigment. Black hair contains only 

 granules and these are dark in color. 

 Red hair contains both granules and 

 diffuse pigment, and these are light 

 colored. There is a difference in distri- 

 bution; in black hair, granules are abun- 

 dant in both cortex and medulla, while 

 in red hair they are found almost exclu- 

 sively in the medulla. 



Next there is a difference in the effect 

 produced when the same genetic factors 

 are introduced into black and red stocks 

 of guinea pigs. The case of most 

 interest here is the effect of introducing 

 grades of albinism. Complete albinism 

 reduces both black and red to white, 

 the dilution factor, an allelomorph of 

 albinism, when introduced into black 

 stock, produces sepia, a dark brown 

 color, which grades into black. No 



