TACTICS IN PIGMENT-CELL RESEARCH 341 



banded, hooded, and otherwise multicolored animals the old animals are not all 

 pigmented? Why in a multicolored animal is not the old animal characterized by all, 

 or almost all, black pigment ? 



Dr. Silvers: Part of the answer may be due to the fact that in the general integu- 

 ment of pigmented rats and mice, melanogenesis only occurs in the melanocytes of the 

 hair bulb. The other melanocytes in the skin do not synthesize melanin. In the 

 guinea pig there is a population of active melanocytes in the basal layer of the epidermis 

 as well as in the hair bulbs. In this animal at the region of transition where black and 

 white areas are juxtaposed, one does find a very narrow band of pigmented skin from 

 which white hairs originate. This area has been secondarily pigmented by a migra- 

 tion of melanocytes from the black area. However, why this process does not continue 

 throughout the life of the animal, I do not know. It may at an imperceptible rate. 

 The rate of spread around a black graft to a white area also falls off asymptotically. 



Dr. Nalbandov: I have two questions. First, what is the origin of the pigment 

 cells in such structures as the testes in chickens, which may be completely black? 

 Second, what is the origin of pigment cells in the uterine region? Neither the testes 

 nor the uterus are of ectodermal origin. Could chickens which may have one black 

 and one yellow testicle or in which each testicle may be two-toned serve as experimental 

 animals for the study of questions in which you are interested, that is, the origin of the 

 yellow and black pigments? 



Dr. Silvers: If the cells you refer to are dendritic in form, then it is almost certain 

 that they originate from the neural crest. Melanocytes may occur in many regions of 

 the body where one might not expect them, for example, in the spleen, parathyroid, 

 and ovary of the mouse. It is possible that during that period of development when 

 melanoblasts are making their way from the neural crest to their definitive positions, they 

 migrate to and through many areas of the animal, but only persist, differentiate, and 

 produce melanin in suitable environments. 



The existence of both black and yellow pigment in the testis of the chicken is very 

 interesting. Unfortunately we do not have any inbred strains of appropriately pig- 

 mented chickens to investigate this, although it might be possible to make appropriate 

 grafts to chick embryos. In the mouse there are two types of melanin, pheomelanin 

 and eumelanin. Although it has not been proved, I am of the opinion that in the 

 agouti mouse a melanocyte can alternate its pigment production, producing eumelanin 

 at one time and pheomelanin at another. In the chicken, however, all the evidence is 

 consistent with the hypothesis that, although each melanocyte has the ability to syn- 

 thesize either black or yellow pigment, once it begins to synthesize one type it cannot 

 change and produce the other. This is a problem which deserves further investigation. 



Dr. Slatis: Dr. Silvers described a very good experiment for distinguishing 

 between infective and migration theories, because a positive result would support the 

 migration theory and rule out the infective theory. The quality that made the experi- 

 ments on transplanting eye discs in Drosophila look so good was that they distinguished 

 between effects due to the genotype of the host and the genotype of the transplanted 



