338 PRINCIPLES OF EMBRYOLOGY 



and these were later found to be separately extractable with suitable sol- 

 vents. They are both of relatively low molecular weight, though probably 

 bound to proteins when they are in the natural state in the living cell. 



Rather little is known about the development of the red pigment, 

 except that it fails altogether in the absence of the normal allele of brown 

 {bw+) and is affected less profoundly by many different genes. Chromato- 

 graphic analysis has quite recently shown that it is in fact really a mixture 

 of a rather large number (about ten) of different components (Heymann, 

 Chan and Clancy 1950) and that certain other fluorescent compon- 

 ents are closely associated with it (Hadorn 195 ifl, Hadom and Mitchell 

 1951). 



The production of the brown pigment provides a good example of a 

 sequence of developmental steps, and is particularly interesting because 

 it has been possible to discover the actual chemical changes involved in 

 some of these. Sturtevant (1932) pointed out that in a fly heterozygous for 

 vermihon (t^/j^+) it can sometimes happen that the v+ chromosome gets 

 lost at a mitosis, and that a patch of cells with the constitution v may 

 appear among the heterozygous eye facets, which are normal in colour. 

 When this happens with most other genes, an abnormally coloured group 

 of cells can be clearly seen, but no departure from the normal wild-type 

 pigmentation is found in patches which can be shown, on other evidence, 

 to be V in constitution. Sturtevant therefore suggested that some substance 

 diffuses into these cells from the surrounding tissue and compensates for 

 the absence of the y+ gene. 



Beadle and Ephrussi developed a technique of transplanting pupal eye- 

 discs into the body-cavity of other larvae, and by suitably choosing the 

 genotype of the host and of the transplant, were able to show that there 

 are at least two diffusable substances (which they rather unhappily called 

 'hormones') concerned in the production of the brown pigment. One of 

 these is produced under the influence of the normal vermillion gene (f +) 

 and is lacking if that gene is replaced by the mutant-vermilhon allele (v), 

 while the other is similarly related to the cinnabar gene. Investigation 

 finally showed that these substances are derived from tryptophane, which 

 is converted first (under the action of v+) into a-oxytryptophane, which is 

 then oxidised (by a reaction for which no separate genetic control has 

 been identified) to kynurenine, which was before its identification known 

 as the v+ substance. This in its turn is converted, under the influence of the 

 cn+ gene into a 'm+ substance , which is probably 3-hydroxy kynurenine; 

 and after this two further steps of reaction, controlled by the normal- 

 cardinal and normal-scarlet genes, intervene before the actual brown pig- 

 ment is formed (Fig. 15.4). 



