THE INSECTS 



143 



removed, or if it fails to evert, the bud on the other side may regulate 

 so as to form more than the half-thorax v^hich is its normal fate. 



Non-mosaic behaviour of a rather different kind is also exhibited by the 

 gonads and genital ducts during the pupal period. Dobzhansky (193 1) 

 first pointed out that if the testes of Drosopliila simiilans, which are nor- 

 mally spiral in shape, fail for some reason to make contact v^ith the ducts 

 which develop from the genital disc, then the spiralisation does not occur, 

 and the testes remain ovoid. Stern (1941) studied the matter in detail, and 

 showed clearly that the ducts induce in the testes the asymmetric growth 

 which leads to the assumption of a spiral shape. In some species of Droso- 

 pliila, the testes normally grow more or less equally in all directions, and 

 thus remain ovoid ; and Stern found that if the larval testis of a species 

 which should develop a spiral gonad becomes attached to the genital 

 duct of a non-spiralising form, then it also fails to become spiral. This is 

 one of the rather few cases in which a species difference is brought about 

 by a difference in an inductive action rather than by being dependent on 

 the nature of the competence of the reacting material. But, as Stem points 

 out, we are dealing here with the transmission of an asymmetric growth 

 stimulus and not with the evocation of histological type of tissue (Fig. 

 8.15). 



Figure 8.15 



Interaction of gonads and genital ducts in Drosophila melanogaster pupae : a 

 shows the outline of the testis at the time it becomes attached to the male 

 duct; h, the normal coiled form it assumes; c, the uncoiled form resulting 



from failure of attachment. (From Stern 194 1.) 



Figure d shows the adult female genital tract in a fly from which one larval 



ovary was removed; the oviduct (arrow) to which no ovary becomes 



attached fails to elongate. (From Pantelouris 1955.) 



