THE MUTATED GENE 35 



or brown soluble pigments both formed by oxidation of dioxy- 

 phenylalanin (Dopa) through an oxidase according to Bloch; 

 they are always deposited in the form of pigment granules of 

 different shapes; (3) The yellow lipochromes, derived from 

 carotins, stain the horny feather cells in solution. (4) With these 

 pigments the structural colors act to produce the phenotype. 

 (a) The Avails of the external cortical cells of the feather rami 

 contain fine air tubes which produce the blue color of cloudy 

 media. The blue is enhanced by a black background; the 

 addition of lipochrome color to both produces green; (b) If the 

 superficial layer of rami and radii splits into fine scales, another 

 optic effect is produced responsible for the bluish grey of pigeons, 

 etc. It is again blued by a background of black; (c) An iridescent 

 effect is produced if the radii containing melanin are broadened 

 (Rensch, 1925), and the thin horny scales above them act as thin 

 lamellae. A study of these colors as controlled by Mendelian 

 genes in the buderigar (Melopsittacus undulatus) has been made 

 by Steiner (1932). In the development of the feather germ, 

 pigment appears first in the so-called dendritic cells, i.e., branched 

 melanophores, which are situated at the basis of the future 

 branches which are formed as rows of cylindrical cells. In the 

 green race, which contains dark pigment (besides lipochrome 

 and the blue effect), simultaneously the cylindrical cells of the 

 young rami produce melanin. In another race with gray wings, 

 which contains less melanin, the dendritic cells contain more 

 melanin, and the cylinder cells less; and in a yellow or white race, 

 still more pigment is contained in the dendritic cells, and none 

 in the cylinder cells. These dendritic cells, however, do not 

 enter the formation of the feather but are later removed when 

 the young feather breaks through. In the three cases, then, the 

 same amount of pigment is formed but is distributed differently 

 upon the feather cells and the impermanent dendritic cells. This 

 may mean either of two things: (1) The chromogen is formed 

 in the dendritic cells and diffuses from these to the feather cells. 

 The time at which oxidase is released means pigment formation 

 and the end of diffusion. Earlier release, then, means less pig- 

 ment for the cylindric cells. (2) The dendritic Cells may reach 

 a threshold for pigment formation from chromogen at an earlier 

 time and thus absorb more of the available chromogen. These 



