PLASMAGENES 



397 



which of the D alleles is present in the nucleus). If the strain is then trans- 

 ferred to 1 8°, the D antigen will eventually disappear, and an S antigen 

 (corresponding to the S allele present) will take its place. It is, of course, 

 difficult to be certain that there was absolutely no S antigen in the 

 cells when they were originally kept at 29-33°, t)ut it certainly seems 

 probable that this was the case, and that the cytoplasmic determinants 

 producing the S antigens have been formed anew under the influence of 

 the S gene when it became activated by the effect of the lower tempera- 

 ture. These antigen-producing determinants therefore seem to provide 

 examples of * gene-initiated plasmagenes'. 



Another important case is that described by Billingham and Medawar 

 (1948, 1950, Medawar 1947, Fig. 18.6). If a patch of black skin from a 

 spotted guinea-pig is transplanted into a white area, it is found that pig- 

 mentation spreads out from the graft into the surrounding area, forming a 

 sort of halo. In mammalian skin, pigment is formed as granules in the 

 cytoplasm of a system of highly branched dendritic cells, the melanocytes, 

 which are present even in white areas, although there the pigment- 

 forming system is inoperative. Billingham and Medawar argue that the 

 spreading of pigmentation which they have studied is due to the 'infection 



Figure 18.6 



On the left, a shaved area of skin on a guinea-pig, into which a graft of 

 black skin had been made 50 days previously: the black patch is due to induced 

 pigmentation, most if not all of the grafted cells having died. On the right 

 is shown the condition 20 days after an immunising graft from the same 

 original donor was made ; the pigmentation is very largely bleached away, 

 although some traces remain in the centre of the area. (After Billingham and 

 Medawar 1950.) 



