1Nevertheless, the remarkable book by A.G. Searle, Comparative Genetics of Coat Color in Mammals, deserves special mention. Because this comprehensive and well-organized monograph concerns the coat-color determinants of all mammals, it serves in many ways as a "companion" to this effort.
2According to Grüneberg ( 1969; see also Dry, 1926) roughly 1% of the dorsal hairs of mice are guard hairs. Among the remaining overhairs, awls usually predominate and auchenes are rare.
3In addition to the melanocytes of the hair bulb, populations of pigmented melanocytes also occur in the ear and tail skin, but not usually in the hairy (trunk) skin of adult mice. Nevertheless, the epidermis of the hairy skin of newborn mice displays numerous dopa-positive melanocytes which increase during the first few days of life but then decline and virtually disappear by 3-4 weeks of age ( Rovee and Reams, 1964; Quevedo et al., 1966; Hirobe and Takeuchi, 1977a). These cells, however, persist in a nonpigmented state ( Reynolds, 1954; Quevedo and J. Smith, 1968) and reappear, i.e., synthesize melanin, when the mice are exposed to ultraviolet light ( Quevedo and McTague, 1963; Quevedo et al., 1967).
4the neural crest becomes differentiated from the lateral margins of the neural plate when the neural tube develops during early ontogeny. It appears initially in the mid-brain region shortly before the neural folds close; subsequently it is found more and more caudally. Its development in more posterior regions is, typically, from the dorsal portion of the neural tube, where cells undergo rearrangement and rapid proliferation. From the resultant rather loosely connected mass, cells migrate in two directions: laterally toward the dorsal portion of the somite and ventrally along the sides of the neural tube. It is from the former group of neural crest cells that melanoblasts arise ( Searle, 1968a).
5In contrast to the melanocytes of the hair bulbs and skin etc., those of the retina are formed from the outer cells of the optic cup. these melanocytes are histologically very distinct from those of neural crest origin and form a simple epithelial layer of hexagonal cells on the outer surface of the retina from which fine cytoplasmic processes run inward between the retinal rods ( Searle, 1968a).