I3O C. L. TURNER. 



occurs in the normal fly. This lack is first evident in the larva 

 where the ocelli are colorless (Fig. 7, Oc.} and the cells of the 

 M.ilpighian tubules are also devoid of color (Fig. 6). In the 

 Malpighian tubules of the mutant fly there are granular bodies 

 which in the normal fly bear the pigment but are here colorless. 

 In the pupal stage the Malpighian tubules go through the same 

 structural changes as in the normal fly but no pigment develops in 

 the granular bodies. Neither do the ocelli of the mutant which 

 up to this time have developed no pigment become colored. The 

 compound eyes of the mutant which are formed at this stage are 

 normal except that they, too, develope no pigment (Figs. 9 and 

 10). In the adult fly the reddish brown pigment is wholly 

 lacking in the compound eyes (Fig. 10, C.E.) and in the Mal- 

 pighian tubules (Fig. 10). 



The other phases of coloration are not affected. The chitinous 

 coloration in the larva, pupa and adult stages cannot be dis- 

 tinguished from that of the normal specimens. The pigment 

 coloration in the chitinous walls of the compound eyes is entirely 

 obscured in the normal fly by the heavier coloration in the facets 

 but in the mutation, because of the lack of the reddish brown 

 pigment in the facets, the coloration of the chitinous interspaces 

 is very prominent (Figs. 9 and 10). The coloration of the hairs 

 of the mutant is identical with that of the normal fly. 



In their reactions to gravity the normal and mutant flies behave 

 alike but there is a marked difference in their phototropic 

 reactions. The normal brown-eyed specimens have a strong 

 positive reaction to light while the white-eyed ones are practically 

 indifferent to light of the same intensity. Strong light has a 

 kinetic effect upon both normal and mutant flies. 



III. NATURE OF THE DETERMINER. 



If a single determiner be postulated for the lack of the reddish 

 brown pigment which appears in turn in the simple eyes of the 

 larva, the Malpighian tubules of the larva, pupa and adult and 

 the compound eyes of the pupa and adult it would seem to be 

 sufficient to account for all phases of the mutation. The fact that 

 the pigment occurs in all three phases or not at all would argue for 

 such a determiner. If any case had arisen in which the reddish 

 brown pigment occurred in one phase and not in another it would 



