THE ROLE OF GENES IN THE EPIGENETIC SYSTEM 337 



control. In hairless, shaven-naked and prickly, both cells lie side by side at 

 the surface and both produce sockets, so that no hair is formed; while in 

 stubhle the tormogen is shifted slightly to one side and sits less firmly on 

 the trichogen, and the hair is thicker and shorter than normal. These 

 phenomena indicate that the shape of the bristle is at least partly due to 

 the moulding of the still-fluid secretion of the trichogen as it is forced 

 through the constricting ring of the tormogen. Several genes affect the 

 nature of this secretion and the rate of its production. Spineless and morula 

 reduce its amount considerably, so that only short and thin bristles are 

 formed, and the secretion of the trichogen is also reduced to the level 

 characteristic of the tormogen in those mutants in which both cells lie 

 side by side at the surface {hairless, shaven-naked, prickly). In stubble the 

 early rate of production is increased but the fmal value is much the same 

 as in normal animals. Some other genes have a more subtle effect. During 

 its secretion the bristle seems to consist of a thin plastic wall and a more 

 fluid core, the whole of which eventually hardens and becomes hollow. 

 The wall is probably formed of an oriented long-chain high-polymer in 

 a rubber-like state, almost devoid of cross-linkages between the chains. 

 It normally grows just at the rate required to provide enough surface to 

 accommodate the fluid secretion which is being forced into it by the 

 trichogen. Tliis delicate balance between growth in surface and in volume 

 is disturbed by certain mutant genes. In singed and forked the volume may 

 suddenly increase too fast (or the surface too slowly) so that the bristle 

 'explodes' and forms bulges and kinks, while in bristle there are gradual 

 and periodic changes in the volume surface/relationship, so that a series 

 of fairly gentle swellings arises. Moreover these genes also effect the mech- 

 anical properties of the surface, which is more plastic in singed sLnd forked 

 and more elastic in bristle. 



Many of these mutant types of bristle may also be produced in pheno- 

 copies, but the correlation between the sensitive periods and the points 

 of attack of the genes is not so fidly worked out as for the wings. 



An even more impressive demonstration of the real complexity of 

 apparently simple developmental processes is provided by the chemical 

 investigations which have been made in recent years on the eye pigments 

 o£ Drosophila (Beadle 1945, Ephrussi 1942, Nolte 1952) and various other 

 insects, e.g. the meal worm Ephestia (Kiihn 1941, Caspari 1949) and the 

 silkworm Bombyx (Kikkawa 1953). In Drosophila at least twenty-five 

 genes are known to affect the colour of the eyes, which normally are a 

 slightly brownish red. Analysis of the effects produced when two 

 different mutant genes are both present in homozygous condition led 

 to the suggestion that there are two main pigments, a brown and a red, 



