INDIVIDUATION— FORMATION OF PATTERN AND SHAPE 427 



IS no sign of any general reaction of the pattern to compensate for 

 the local absence. 



The last-formed element in the pattern— the posterior cross-vein— 

 always adjusts its position in relation to any previous event which has 

 affected its general neighbourhood. For instance, if the longitudinal veins 

 are splayed apart at their first inception, e.g. by dachs, or even by differ- 

 ential growth immediately after their formation, e.g. in the mutant 

 broad, then the posterior cross-vein moves inwards towards the base of 

 the wing. It reacts to absences of the distal parts of the longitudinal veins 

 L4 and Ls even when these are produced by genes acting as late as the 

 pupal contraction, such as veinlet. It can also be moved by surgical opera- 

 tions made at a similar time (Lees 1941). These variations in the position 

 in which the posterior cross-vein appears make it very obvious that the 

 pattern is an expression of an equilibrium resulting from the interaction 

 of numerous factors. 



Two other observations on wing venation may be mentioned, to 

 emphasise the complexity of the underlying epigenetic processes and the 

 equilibrium character of the simple pattern which is normally produced. 

 Timofeeff-Ressovsky (193 1) studied a gene Vti {venae transversae incom- 

 pletae) in Drosophila funebris. This causes a break in the posterior cross- 

 vein. He found it possible, by selection, to isolate different stocks, in one 

 of which the break occurred at the anterior end of the vein, in another 

 at the posterior end and in the third at both ends with equal frequency. 

 The genetic differences between these stocks depended on numerous 

 factors of small effect. Since each of the genes concerned is presumably 

 producing its own specific effect, we must conclude that very many 

 mdividual processes are involved in this very detailed determination of a 

 small part of the venation pattern. Again, another gene in the same species, 

 also studied by Timofeeff-Rissovsky (1934), produces an effect which 

 can be considered as a general disturbance of the condition of equihbrium 

 of the vein-forming processes; the effect is either that many parts of 

 the vein are missing or that large amounts of extra vein material are 

 formed. 



The thoroughness with which the venation pattern can be analysed, 

 thanks to the large number of mutant forms available, has revealed a 

 number of general points which are probably applicable to other cases of 

 animal pattern about which we have as yet less actual knowledge, hi the 

 first place we see that even a comparatively simple pattern, such as that of 

 the Drosophila wing venation, may be composed of a number of different 

 parts which arise relatively independently and in diis case at somewhat 

 different times. Thus the main longitudinal veins L3, L4 and L5, pass 



