TACTICS IN PIGMENT-CELL RESEARCH 333 



of the pigment granules present in the hair septules in various genie substitutions to 

 determine the effects of individual loci on these attributes. 1092, 1093, 1094, 1095 Studies 

 utilizing tissue-culture methods such as those described by Cohen 210 and Reams, 

 Nichols, and Hager 1042 in the chick might also be profitable with these coisogenic 

 strains. 



These stocks also provide genetically uniform material required for investigating 

 the pleiotropic effects which many of the genes associated with pigmentation have in 

 other tissues. Among these effects are those which involve hematopoietic tissues, 

 absorption of bone, eye size, formation and function of the nervous system, and number 

 of germ cells. 499 



PROBLEMS OF PIGMENTATION IN SPECIES OTHER THAN MICE 



Despite the availability of large numbers of genetically uniform coat-color types 

 of mice and their obvious experimental advantages for pigment-cell genetics, there are 

 certain pigmentary phenomena which can only be investigated in the particular 

 species in which they occur. Among these is partial albinism found in Himalayan 

 rabbits and certain other mammals of similar phenotype. In this condition the fur is 

 predominantly white but pigmented at the extremities of the body. However, it has 

 long been known that pigmented hair appears in other normally nonpigmented regions 

 of the body after shaving the skin and maintaining the animal in a cold environment. 1177 



The intriguing tortoise-shell (e p e p ) pattern in the guinea pig, which is characterized 

 by an irregular intermingling of yellow with black hairs, and the effect which spotting 

 has on this pattern is also without a counterpart in the mouse and certainly deserves 

 investigation. In the presence of spotting factors, especially ss, not only is there an 

 increase in the amount of yellow in tortoise-shell genotypes but, in addition, in e p e p ss 

 animals there is a tendency toward segregation of yellow and black so that a tri- 

 colored yellow, black, and white spotted phenotype results. 199, 1419, 1449 



Pigment spread. — Finally the provocative phenomenon of pigment spread as it 

 occurs in guinea pigs, Friesan cattle, and other mammals 100 is of interest to those 

 concerned with the potentialities of the melanoblast and melanocyte. If black skin 

 from a black-and-white-spotted guinea pig is grafted to a white area on the same 

 animal, there is a slow centrifugal encroachment of pigmentation into the white skin 

 from the graft margins. Conversely, if white skin is grafted into a black area, it 

 gradually blackens from its periphery inward. This spread of pigment occurs from 

 black into yellow (red) or from yellow into white skin, and it also occurs naturally 

 where skins of two different colors are juxtaposed on the same animal. 



Although this phenomenon has been investigated repeatedly, no completely 

 satisfactory or generally acceptable explanation of the underlying mechanism has yet 

 been forthcoming. The two most attractive hypotheses which have been advanced to 

 explain this phenomenon are: (1) it results from a transformation of amelanotic 

 melanocytes into melanin-producing cells or of phaeomelanin-producing into 



