THE INSECTS 



141 



of the mesothoracic one, a result similar to that produced by the gene 

 bithorax. 



It was for some time thought that the determination of the imaginal 

 buds which occurs in mid-embryonic life was the final step which fixed 

 the fate of each part, converting the buds into a rigid mosaic. For in- 

 stance Bodenstein (1941) found no signs of regulation when limb-buds 

 from third instar larvae were halved. However, Waddington (1942^, h) 

 showed that the determination is by no means fmal even in larval stages 

 (Fig. 8.13). If early third instar larvae are given a heavy dose of x-rays, 



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'W^-^^mf r''i'\ ^W"^^"^ 



Figure 8.13 



Regulative development in Drosophila. On left, a vestigial fly in which one 

 mesothoracic bud has failed to evert and the other produced considerably 

 more than half a thorax. (From Waddington 1953.) On right, conversion 

 of eyes into palps following x-raying of the late larva. (From Waddington 



many of the imaginal bud cells are killed, and those that remain may pro- 

 duce duphcated organs, or even something quite foreign to their normal 

 fate (e.g. eyes in place of antennae or vice versa). Various authors then 

 found that when the imaginal buds of the larva are cut into fragments 

 which are allowed to develop in isolation, their behaviour is not strictly 

 mosaic. Hadom and Gloor (1946) showed that the female genital bud be- 

 haved like a series of overlapping fields (Fig. 8.14), and Hadom, Bertani 

 and Gallera (1946) found that in the genital bud of the male considerable 

 reorganisation and regulation of these fields is possible. Vogt (1946^) ob- 

 tained very similar results with the eye-antennal bud. The same author 

 (1946^) studied the development of eye-antennal buds of flies homozygous 



