32 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 1 3/ 



abdominal cavity of a green mutant host brought about pigment differ- 

 entiation, not only in the host eye but also in the host testes. As a 

 matter of fact, the diffusible substance necessary for eye pigmentation 

 seems to be the same as that for testis pigmentation, because the vari- 

 ous organ grafts that produce the diffusible substance and thereby 

 elicit pigment formation in the host eyes were also effective in 

 causing the formation of pigment in the testes. The evidence suggests 

 that both eyes and testes of the green-eyed Musca mutant depend on 

 the tryptophane metabolite kynurenine for completing successfully the 

 biosynthesis of the brown pigment. One will notice from table 2 that 

 there is an important difference between the eye and the testis as far 

 as color development is concerned. The cells of a wild Musca eye disc 

 are able to synthesize kynurenine or a kynurenine-like substance ; an 

 eye disc, therefore, always develops its brown color autonomously if 

 grown in a green mutant host. This is not the case with the testis. A 

 wild-type testis transplanted into a green mutant host remains color- 

 less. The color development of a wild-type testis is a nonautonomous 

 process, for it depends on a precursor substance produced by other 

 wild-type tissues. In this respect, mutant and wild-type testes are 

 alike. 



EXPERIMENTS ON PERIPLANETA AMERICANA 



Studies on the biosynthesis of the tryptophane-derived brown eye 

 pigment, using eye-color mutants as a tool for the analysis of pigment 

 formation, have so far been conducted only on holometabolous in- 

 sects. The major and final steps leading to the actual synthesis of pig- 

 ment in the pigment-carrying eye cells take place in these forms dur- 

 ing the differentiation of the imaginal eye in the pupal stage. In 

 hemimetabolous insects, which already in their nymphal stages possess 

 a fully differentiated compound eye, the course of events must be 

 different. The formation of eye pigment must have occurred here dur- 

 ing embryonic development. Therefore, it might seem useless to at- 

 tempt the induction of pigment formation in the nymphal eyes of 

 these creatures, because the metabolic events responsible for eye pig- 

 mentation must have already run their course at this stage. But this 

 is not the case. 



The nymphal eye grows from instar to instar by the addition of new 

 facets from a so-called budding zone. In this zone, during the inter- 

 molt periods, the formation of new imaginal eye elements occurs, much 

 the same as it does in the eye disc of a fly during pupal life. The 

 budding zone must be considered the prospective eye anlage for the 



