THE REALISATION OF THE NUCLEAR FACTORS 89 



takes care of the production of the v + substance has fallen out. 

 In "cinnabar", the v+ substance is produced, but it cannot be 

 transformed into the cn+ substance because of the absence of 

 the necessary gene (Fig. 31). As we have seen, these substances 

 are produced by certain cells of the body under the influence 

 of certain genes, and then spread throughout the body via blood 

 or lymph, so that they satisfy the definition of gene hormones. 

 A single gene hormone may influence several different devel- 

 opmental processes. In Ephestia, one substance influences the 

 speed of development, and the viability of the embryo, the 

 pigmentation of eyes and skin of the caterpillar, and the 

 pigmentation of eyes, brain, and sex organs of the imago. 

 Another remarkable discovery is that gene hormones are not 

 specific. The v+ and cn+ substances of Drosophila mela- 

 nogaster were also found in other flies, parasitic wasps, and 

 butterflies. The catenary reaction found in Drosophila appar- 

 ently occurs in insects in general, and the existence of muta- 

 tions entirely analogous to ''vermilion" and "cinnabar" in 

 Drosophila has been demonstrated in Ephestia and Habrobracon^ 

 where, as a matter of fact, they had been known for a long 

 time under different names. On the other hand, the end point 

 of the reaction is not always the same. There is a wide variety 

 of eye pigments in different insects. Moreover, the pigments of 

 eyes and skin in Ephestia are not identical, though the pro- 

 duction of both is governed by the same gene. Evidently, the 

 gene substances do not influence the last stages of pigment 

 formation, but instead they constitute a link in earlier processes. 

 Later work has thrown some light on the nature of these processes. 

 The v-\- substance was found to be identical with kynurenin, 

 a derivative of tryptophane. The cn+ substance is a derivative 

 of kynurenin. Apparently, the genes exert their influence 

 through the production of enzymes which first transform 

 tryptophane — itself a common intermediate in protein meta- 

 bolism — into kynurenin, and subsequently kynurenin into 

 other substances which are the building stones for skin and eye 

 pigments. In this case, therefore, the name gene hormone may 

 not be entirely appropriate, because the substances concerned 

 are not themselves catalysts, but building material that is 



