,;i PHYSIOLOGICAL GENETICS 



As far as this work goes, it proves that the embryological differ- 

 ence between the different mutants consists in differences 

 (1) in time of onset of the process of pigmentation; (2) in 

 the amount (or rate?) of oxidation and reduction; and (3) in the 

 pattern of deposition of pigment (probably also in the size of the 

 granules). Such a combination of reactions of different rate 

 as found here had actually been postulated by Goldschmidt 

 (19206, 1927c) as a basis for the explanation of this and similar 

 cases. 



As a supplement to the results on the Drosophila eye, the facts 

 that Wolsky and Huxley (1934, 1936), described for the develop- 

 ment of eye mutants in Gammarus are of importance. There 

 are mutants in eye structure in which the retinula is absent 

 or reduced to scattered cells, the crystalline cones are defective, 

 and the eye is composed mostly of hypertrophied connective 

 tissue. Furthermore, the optic tract is affected (which is also 

 the case in the Drosophila eye mutants). The development of 

 these eyes is abnormal from the outset: the retinula cells arc 

 deficient in number, do not form regular groups, and later 

 degenerate. Simultaneously the interstitial cells hypertrophy. 

 The centrifugal differentiation of the optic tract with its three 

 ganglia is delayed and inhibited. The adult tract then resembles 

 a developmental stage of the normal eye. As far as this descrip- 

 tion goes it seems that this development is comparable to the one 

 of the vestigial mutant; very early probably the Anlage is normal, 

 but early in development degeneration sets in, beginning in the 

 distal region of the organ. From these facts we may expect 

 that a histological study of the early stages of the Drosophila eye 

 mutants might reveal similar conditions and not simply a retarded 

 growth. 



Some of the phases of the process of pigmentation as dependent 

 upon mutant genes seem to be more easily understood in birds. 

 The pigmentation of the feather and the general basis of feather 

 colors has been the subject of numerous researches of which we 

 mention especially the school of Haecker, because they worked 

 with genetical ends in mind (Haecker, 1918, 1925; Goernitz, 

 1923; Kniesche,' 1914; Spoettel, 1914). In a general way, the 

 different colors are produced by a combination of (1) Eumelanins 

 = dark relatively insoluble pigments ; (2) pheomelanins = yellow 



