426 PRINCIPLES OF EMBRYOLOGY 



square in shape, and it seems probable that it is the change in wing shape 

 which has brought about the akeration in the laying down of the 



pattern. 



It is not quite clear what happens to the pattern if the size of the wing 

 is reduced, without alteration of its shape, before the venation is deter- 

 mined; genes which produce small wings (such as miniature) act largely, 

 if not wholly, after the pattern has been laid down and thus, like dumpy, 

 distort something which is already in existence rather than alter the con- 

 ditions under which it comes into being. Genes are known, however, 

 which cause increases in the wing mass earher than the period of pattern 

 formation. One is dachsous; this usually produces a fairly slight increase in 

 size and a five-rayed pattern of longitudinal veins appears, the angle of 

 divergence being, of course, larger than usual. In hlot, on the other hand, 

 the exaggeration in size is greater, and extra longitudinal veins make their 

 appearance. In extreme cases these extra veins can be seen to form a 

 mirror-image of the normal venation, the mirror plane being along the 

 position of L5. Such duplication and mirror-imaging is rather common 

 when a pattern is developing in a mass of tissue which is, as it were, too 

 large for it. One feels that it should offer an important clue as to the nature 

 of the essential processes concerned in pattern formation, but so far no 

 one has suggested just how it should be interpreted: stimulating discus- 

 sions are given by Harrison (i945). Needham (1936^). 



The whole set of five longitudinal veins does not, however, always 

 behave as a unit. There are certain genes which have locaHsed effects on 

 particular veins or particular parts of veins. House (1952) has been able 

 to show that some genes which appear to have strictly localised effects 

 may exert on neighbouring regions sub-threshold influences which are 

 not strong enough to produce any actual alteration except in combmation 

 with other genes; but even if this is the case, it remains true that these 

 genes affect most strongly particular sections of the venation rather than 

 the system as a whole. One gene of this kind which acts at an early stage 

 is cubitus interruptus. This causes an absence of the distal end of L4 in the 

 prepupal stage, and this vein does not reappear and is also missing m the 

 adult. It is interesting to find that in the aduk wing the tip of L5 moves 

 forwards, as though to try to fill the space left by the absence of L4. We 

 have, then, a certain reaction of the pattern as a whole to the local defect 

 which is the primary effect of the gene. This reaction of the whole system 

 probably occurs at the second phase, that of the pupal contraction, the 

 absence of L4 having been produced earher, in the prepupal phase. Most 

 local absences of veins, such as those caused by veinlet, tilt or radius 

 incompletus, do not occur until the second phase, and in these cases there 



