CELLULAR DIFFERENTIATION AND INTERNAL ENVIRONMENT 



91 



with both amphibian and chick material. 

 While the recorded observations deal 

 mainly with melanophores, the above prob- 

 ably applies also to the xanthophores, 

 which contain a diffuse yellow lipochrome 

 and to the guanophores containing granular 

 pigment with metallic luster. 



When the cells of the neural crest start 

 out from their dorsal position, the potential 

 chromatophores are indistinguishable from 

 their accompanying cells. They are at first 

 without pigment and reach remote positions 

 in the embryonic body before showing their 

 differentiation. There is evidence that 

 many of them are not self -differentiating 

 but require activation by something derived 

 from other cells in order to produce pig- 

 ment (DuShane' 1935; Harrison 1938). 



The above facts are shown by simple ex- 

 periments. When the neural crest is re- 

 moved bilaterally from the trunk region of 

 an amphibian embryo, the whole flank re- 

 mains without pigment. When neural crest 

 is transplanted to the abdominal region, 

 much pigment develops and spreads from 

 the site of the graft. Neural crest explanted 

 in vitro develops many pigment cells 

 (Dorris 1938 ; Twitty and Bodenstein 1939). 



Pigmentation is frequently not diffuse 

 but is distributed according to patterns 

 that have definite relations to organs and 

 regions. By means of transplantation ex- 

 periments with embryos of three closely re- 

 lated species of Triturus and their hybrids, 

 Twitty (1936) has shown that such mark- 

 ings are the result of interplay of a multi- 

 plicity of factors, some inherent and some 

 environmental.^ The latter may be either 

 of the intimate cell-to-cell type or humoral. 



The stripe along the dorsal margin of 

 the myotomes characteristic of larvae of 

 Tr. torosus is not formed when neural crest 

 of Tr. rivularis, which does not have stripes 

 but a much more extensive diffuse pigmen- 

 tation, is grafted in place of the neural 

 crest of the torosus embryo. Keciprocally, 

 neural crest of torsus forms stripes on riv^i- 

 laris. However, the stripes do not form in 

 the absence of the myotome ridge in either 



7 Op. cit., p. 18. 



8 See also Twitty and Bodenstein (1939). 



species. Both grafting and tissue culture 

 experiments show that the rivularis neural 

 crest cells tend to spread much farther 

 from their seat of origin than do those of 

 torosus. This seems to be correlated with 

 the slowness of rivularis cells to differenti- 

 ate pigment as compared with cells from 

 torosus. Movement and differentiation are 

 in inverse relation to one another. 



In the amphibian embryo the pigmentary 

 system is the first tissue that can be ob- 

 served to respond to hormonal action 

 through the medium of the blood. When 

 the hypophysis rudiment is removed in 

 early embryonic stages, the melanophores, 

 which begin to differentiate about the time 

 the circulation of the blood starts, do not 

 expand. The larvae acquire a silvery or 

 yellow appearance, according to species, 

 owing to the preponderance of other pig- 

 ment-bearing elements. This effect is pro- 

 duced primarily through the distribution 

 of the pigment within the individual 

 melanophores and in this respect is like the 

 adaptive color changes in organisms in re- 

 sponse to temperature, light and color of the 

 background. However, it has been shown 

 that continued absence of the hypophy- 

 sis results in the disappearance of some 

 of the non-melanophore pigment in the epi- 

 dermis, just as the presence of more than 

 the normal amount (by grafting super- 

 numerary hypophyses) produces an organ- 

 ism with more than the normal amount of 

 pigment, due partly to "expansion" of the 

 melanophores and partly to increase in the 

 non-melanophore pigment in the epidermis 

 (Blount 1932). The pigmentary system 

 thus may participate in more rapid changes 

 of a physiological nature and in slower dif- 

 ferentiating changes that may be more or 

 less permanent, both taking place as a re- 

 sult of hormonal action. 



In manj^ organisms the pigment pattern 

 is very sharply defined and highly specific, 

 often in great detail, and in certain fishes, 

 amphibians and birds it is known to be as- 

 sociated with sex and sexual activity 

 through mediation of hormones. Experi- 

 ments have also shown that other endocrine 

 secretions and related substances, e.g.. 



