THE ROLE OF GENES IN THE EPIGENETIC SYSTEM 335 



build up Strains whose hereditary constitution predisposes them to 

 respond in particular ways to any given treatments. This specification 

 of the developmental stability of the organism is surprisingly precise and 

 detailed; it usually depends, in any given case, on rather a large number 

 of genes; and starting from any fairly large population, it seems to be 

 always possible to find in it genes which will confer on their bearer 

 almost any type of developmental reactivity one chooses. These facts 

 again bring to our attention very forcibly the complexity of the genetic 

 system which controls developmental processes. (For discussion of the 

 evolutionary implications of phenocopy-formation and similar pheno- 

 mena, see Waddington 1953 &, 1954&.) 



We may now turn to another example of a developmental sequence 

 which has been well analysed from a genetical point of view (Lees and 

 Waddington 1942, Lees and Picken 1945). The formation of the bristles 

 (macrochaetae) in Drosophila appears, by histological investigation, to be 

 an extremely simple process, directly involving only two cells per bristle^ 

 (Fig. 15.3). In the pupae of about fifteen hours' age slightly enlarged cells 

 may be found in the hypodermis. Aheady they are sometimes in pairs, 

 though they may also occur singly. By nineteen hours they are always 

 paired, and it is probable that a division, producing a pair of cells, occurs in 

 the period from about twelve to eighteen hours. The two cells proceed to 

 grow rapidly, attaining a volume about a thousand times as great as that 

 of the neighbouring hypodermal cells. During this process the nuclei 

 enlarge greatly, and the chromosomes assume the polytenic banded form 

 which is best developed in the salivary gland cells. The pair of cells be- 

 come arranged in a characteristic way, with one lying above and slightly 

 to one side of the other. The upper cell, which is known as the tormogen, 

 is destined to form a more or less circular socket of hardened chitinous 

 material, while the lower one, the trichogen, produces a long gradually 

 tapering bristle which sticks up through the centre of the socket. A whole 

 series of genes affects this relatively simple sequence of processes. In the 

 first place there are some, such as scute, which cause an absence of certain 

 of the normal bristle cells, and others, such as hairy, which produce extra 

 pairs; nothing further is knowoi about their mode of action. The next 

 stage, that of the cell division to produce the trichogen-tormogen pair, 

 is affected by split, which often provokes an extra division to give a 

 group of four cells; these become somewhat irregularly arranged, and 

 may give rise to two trichogens and two tormogens or only one of the 



^ There are one or more other pairs of cells closely associated with these ; a general 

 description is given by Henke (1953) • 



