23O BENJAMIN HORNING AND HARRY BEAL TORREY. 



The melanins appear in definite chromatophores, being formed 

 in situ. The chromatophores are found in the epidermis of the 

 rachis and barbs. At first they are lacking in pigment which is 

 gradually laid down within them in the form of granules. The 

 pigment of the barbule cells is obtained by a direct transfer 

 from the chromatophores which send out amoeboid processes as 

 the barbules develop, these processes meeting and fusing with the 

 barbule cells as though guided by a tropistic factor. Readers 

 who are interested in the details of this remarkable mechanism 

 are referred to the papers of Strong (1902) and Lloyd Jones 



(I9I5)- 

 The expansion of melanins in the feathers of thyroid fed 



birds is conceivably dependent, then, on several factors: an 

 increased amount of melanin pigment ; an expansion of individual 

 melanophores, with or without added pigment; an increase in 

 number of melanophores; a migration of melanophores. Con- 

 sidering the large areas ordinarily free of melanins that darken 

 under thyroid feeding, it is difficult to avoid the conclusion that 

 the total amount of melanin is increased. The extension of 

 individual melanophores over very much smaller areas is certainly 

 a fact. Whether or not the melanophores multiply we have not 

 determined. The number of functional melanophores assuredly 

 increases. We have no evidence of the migration of melanophores 

 over the long distances required by the observed facts, and it 

 appears to be highly improbable. 



The darkening of feathers as a result of thyroid feeding is 

 probably due chiefly, then, to an increase in melanin pigment 

 and an increase in the number of melanophores functioning 

 with or without an increase in the number of cells themselves. 

 Thyroid feeding is thus conceived to promote pigment formation, 

 directly or indirectly, and possibly cell division as well. These 

 are two sufficiently diverse functions, both, in our opinion, 

 indirect. Whether the melanophores do or do not multiply under 

 thyroid influence, there is no doubt that other cells do, namely, 

 the cells of the barbules whose number and extent is so markedly 

 increased. But it is not clear at present in either case through 

 what channels the thyroid acts to augment pigmentation or cell 

 proliferation. Evidence has been given elsewhere (Torrey, 



