GENE CONTROLLED PROCESSES I79 



unable to react to the substances even if supplied, since these types, 

 unlike v and en eyes, do not acquire normal pigment when implanted 

 into wild-type larvae. There is no obvious relation between the groups 

 of genes classified together in this way and the groups derived by 

 Schultz; perhaps future work will reveal the connection. 



The ca substance is of rather a different nature, since no eye can make 

 its own supply but must obtain the substance from some other part of 

 the body. Thus a wild-eye disc implanted into a ca larva does not 

 obtain ca substance and therefore cannot manufacture either v or en 

 substances and develops a claret colour. 



The V and en substances occur in other inseas as well as Drosophila, 

 and extraction experiments have been made on the lymph of Calliphora 

 pupae. The active substances are water-soluble and contain nitrogen 

 and are probably related to the purines. These substances seem to 

 affect not only the colour in the eye but also the number of facets 

 formed (p. 184). 



Another pigment system which has been fairly fully investigated is 

 that of the coat-colours of rodents. The pigments are melanins formed 

 probably from amino-acids such as tyrosin. Wright^ has argued that all 

 the colours can be explained by invoking two enzyme reactions: 

 Enzyme I, acting on the fundamental substrate amino-acids (or chro- 

 mogen compounds), gives a yellow pigment, while enzyme II has no 

 effect alone, but when combined with I forms a sepia pigment, even 

 from concentrations below the threshold for the action of I. Some 

 genes act by suppressing or partially inhibiting the action of I and thus 

 affect all colours (e.g. the albinism series of hypomorphs weakens the 

 action of I while dominant white (neomorph) inhibits it altogether by 

 the presence of an antienzyme), others affect only the black or sepia 

 pigment-formation due to II. 



In all the cases so far mentioned, the gene-controlled substances 

 are only observed after a long series of developmental processes has 

 occurred, and it is not by any means clear that these end-products are 

 directly produced by the genes concerned; it is therefore not safe to 

 assume that the nature of the observed substances gives any clue as to 

 the nature of the genes. In a few special cases, however, the develop- 

 mental path between gene and substance is much shorter. The best 

 cases are in gamophase charaaers of higher plants. ^ For instance, in 

 maize a plant heterozygous for waxy^ produces two kinds of pollen, in 



* Wright 19 1 7, 19 1 8, 1927, for transplantations, cf. Reed 1938. 



^ Cf. Stem 1938. 8 Brink 1929. 



