TACTICS IN PIGMENT-CELL RESEARCH 335 



To establish whether the melanocytes in the annulus of pigment spread around a 

 black-in-white graft are of graft origin or not, a crucial point for the migration hypo- 

 thesis, Billingham and Silvers are currently utilizing two isogenic strains (as con- 

 firmed by skin-grafting procedures") of guinea pigs and their F 1 hybrid. These 

 strains 2 and 1 3 are of the e p e p ss genotype (described above) and, therefore, can each 

 provide and receive black grafts to initiate pigment spread in initially white areas. 



The experimental procedure represented diagrammatically in figure 46 is as 

 follows. Pigment spread is initiated in white spotted areas of F x hybrid hosts by 

 means of black grafts originating from histocompatible parental strain animals. The 

 specific techniques for carrying out these experiments have been described by Billing- 

 ham and Medawar. 95, 96 After a period of not less than 100 days, when there is a 

 sufficient annulus of pigment spread available, an attempt is made to determine 

 whether the genotype of the melanocytes present in this annulus is of the donor strain 

 or of the F 1 hybrid. This is achieved by isolating pigmented epidermis from the 

 annuli of spread and determining the ability of the melanocytes contained therein to 

 survive in specifically sensitized animals (in respect to F x hybrid homografts) of the 

 donor strain. 



If pigment spread is simply the outcome of melanocytic or melanoblastic migration, 

 then the melanocytes of the graft and those that surround it in the annulus of spread 

 must be of the same genotype : that of the donor. They should, therefore, survive 

 when transplanted back to the donor strain. If, however, an infection process is 

 involved, then only the melanocytes from the graft should survive, since those from 

 the annulus of spread would be of hybrid genotype and consequently should be des- 

 troyed very soon after they are placed on the presensitized, secondary host. 



CONCLUSION 



An attempt has been made to discuss only some of the many methods which can 

 be employed in studying the genetic aspects of mammalian pigmentation. Most of 

 these methods have involved tissue transplantation, since this technique has proved to 

 be of utmost importance in approaching experimentally many of the problems of 

 pigment-cell workers, particularly those concerned with the physiologic genetics of 

 pigmentation. 



It is obvious that the progress which has been made in understanding how genes 

 control such processes as melanoblastic differentiation and synthesis of melanin has 

 rested, first, upon the availability of color stocks uniform with respect to at least those 

 hereditary factors concerned with pigmentation, second on the production of inbred 

 color stocks, and finally on the existence of coisogenic color lines. Although almost all 

 of these have been established in either the guinea pig or the mouse, mainly through 

 the pioneering efforts of Drs. Sewall Wright and C. C. Little, they include such a wide 

 range of different phenotypes that among these two species can be found suitable 

 subjects with which to investigate almost all aspects of pigment-cell biology. There is 



