LIMB FIELD OPERATIONS 283 



pigment; and Guanophores, with the metallic, gold or silver, guanin. The pigmentation of 

 a transplanted limb always resembles that of the host, except in transplants involving the 

 white axolotl (Harrison, 1935). There are microscopic differences not only in color of 

 pigment cells, as suggested ahove, hut in their size and shape. Incidental to part "A" of 

 this experiment, the effect of the host on the pigment of the donor limh can be determined 

 in the A. tigrinum and A. punctatum transplants. If the rare white axolotl embryos are 

 available, this experiment would most graphic and significant. 



The pigment cells are derived from the ganglion crest cells which migrate toward the 

 limbs in the earliest motile stage of the embiyos (DuShane, 193'+). A few xanthophores and 

 melanophores are developed when ectoderm from a normally pigmented embryo is grafted to 

 the limb bud of a white form. This suggests that the white axolotl has the melanophores 

 but that the ectoderm of the white axolotl lacks some of the activating principle which is 

 found in the ectoderm of the pigmented species. DuShane suggests that the ganglion crest 

 gives rise to cells which normally develop pigment and that a second factor (ecto- or 

 mesodermal) is necessary to activate the process. Periclinal and sectorial chimeras often 

 appear. (See section on "Neural Crest Origin of Pigment".) 



EECOED OF GENETIC FACTOES IN LIMB DEVELOPMENT 



