332 PRINCIPLES OF EMBRYOLOGY 



into folds which only become flattened out again after the imago emerges 

 from the puparium. 



The final wing is a simple-enough organ. It is practically two dimen- 

 sional, since the thin upper and lower surfaces fuse tightly together; its 

 outline is a simple oval slightly indented where the most posterior vein 

 cuts it; and the whole system of venation consists only of one vein forming 

 the fore-edge and four main longitudinal ones radiating from the base 

 of the wing with two short cross-veins between them. 



The wing path of development is affected by very many genes ; Wad- 

 dington (1940&) has described the abnormalities produced by some 

 thirty of them, and a fair number of others are known. The mutant alleles 

 of these genes are recessive to the wild-type alleles ; that is to say, the 

 epigenetic path is canalised to the extent that an alteration of only one of 

 the two alleles to the mutant form does not suffice to produce any notice- 

 able alteration in the course of development, presumably because some 

 threshold is not exceeded in the heterozygote. 



A few of the recessive forms are illustrated in Fig. 1 5 . i , which shows 

 how their development diverges from the normal pathway. Each step 

 in the normal sequence is influenced by genes, which often act upon the 

 developing tissues in opposite directions. For instance, in the prepupal 

 wing, the relative rates of cell division in different directions are affected 

 by the genes broad, expanded, lanceolate and narrow, of which the first two 

 cause the wing to become broader and the last two longer. Again, the 

 time of the pupal contraction is a minor epigenetic crisis, during which the 

 contracting wing is in a state of dehcate balance, influenced by genes 

 such as dumpy, humpy and spade, which tend to increase the contraction in 

 length, hlade which tends to increase it in width, balloon and bloated yAi\c\i 

 tend to reduce the contraction in general; while genes whose primary 

 effects are to change the shape of the wing margin may produce secondary 

 effects at this time, since if the wing is abnormally long and narrow, or 

 short and broad, before the contraction starts, these characteristics will 

 become exaggerated. During the later stages of the contraction, the 

 imagitial veins appear as cavities remaining between the two wing epith- 

 elia and there are some genes, such as veinlet, tilt, radius incompletus, which 

 tend to cause obliteration of veins, while others, such as plexus or net, work 

 in the opposite sense, and produce extra veins. 



From a study of this large number of genes affecting the development 

 of a single organ, a picture of the general epigenetic situation emerges. 

 Direct investigation by more conventional experimental techniques has 

 confirmed it in many points. Thus Lees (1941) made defects in the develop- 

 ing wing by pricking it with a needle at various defmite times, and could 



