424 PRINCIPLES OF EMBRYOLOGY 



3. Some actual patterns 



It is now time to turn from this somewhat abstract discussion of the 

 fundamental principles of pattern formation to the consideration of one 

 or two actual examples of patterns which have been relatively fuUy 

 analysed. Studies employing essentially biological methods (e.g. investi- 

 gation of the development of mutant types, or the performance of surgical 

 operations) have shown that many apparently simple patterns result from 

 the interplay of numerous factors. Investigations by chemical or physical 

 methods have not as yet progressed nearly so far. Indeed in most cases we 

 have no actual evidence at all as to the physico-chemical nature of the 

 processes involved and can hardly proceed beyond such a priori arguments 

 as those discussed above. 



(a) Drosophila wing venation 



It is, as might be expected, in Drosophila that genetic methods have 

 provided us with the greatest mass of information concerning patterns. 

 Most of the important principles which emerge from such studies can be 

 illustrated by a consideration of the venation of the wings (Waddington 

 1940b). The veins of the adult wing present a fairly simple pattern, con- 

 sisting of five.longitudinal veins running from base to tip, with two cross- 

 veins, an anterior one between L3 and L4 and a posterior between L4 and 

 L5. This pattern arises in a series of stages, of which we may distingmsh 

 three: (i) The prepupal stage, in which L2 is absent, L3 and L4 are umted 

 from the base to near the middle of the wing blade, there is a marginal 

 sinus right round the edge (part of which corresponds to the later Li) and 

 there are no cross-veins. At the end of this stage the wing is inflated mto 

 a balloon-like shape by the pressure of the internal fluid and all visible 

 traces of the prepupal venations disappear. (2) When the wing contracts 

 again, in the pupal period, the five longitudinal veins make their appear- 

 ance. (3) In a sHghtly later phase, at the very end of the contraction, the 

 two cross-veins appear (Fig. 15. i, p. 33 1)- 



Some genes have effects which affect the five-rayed pattern of the 

 longitudinal veins as a single organised unit (Fig. 20.4). The effects of these 

 generaUy acting genes are, however, of several different kinds. Perhaps 

 the most striking, but the least illuminating with regard to pattern forma- 

 tion, are genes such as dumpy which, by affecting the pupal contraction, 

 distort the pattern of vehis after it has been laid down. This is not really 

 an effect on pattern formation, but only on the expression of the Pattern. 

 Some other effects, however, are more radical. For instance, in shifted ^W 

 the veins from L2 to L5 appear as though squeezed together, divergmg 

 at a lesser angle. This occurs without any noticeable change m the outUne 



