^.Q PRINCIPLES OF EMBRYOLOGY 



mass of tissue turns either into a leg or into a wing say, but it is difficult 

 to persuade it to become something intermediate And also each path i 

 'canalised', or protected by threshold reactions, so that if the developmen 

 IS mildly disturbed it nevertheless tends to regulate back to the normal 

 end-result (Waddington 1940^). 



As a first example in which these prmciples may be seen, we may con- 

 sider the development of the wing in Drosophila (Goldschmidt, 193 5^, 

 1937, Waddington 1940^). The main features are shown m Fig. 15.1 m 

 the form of somewhat diagrammatised drawings of the whole wmg at 

 various stages during its development in the pupa. In the larva at the time 

 of puparium formation, the wing is a thickened area of the dorsal meso- 

 thoracic buds, an area which is already folded in towards the mterior of he 

 bud Very shortly afterwards, the fold elongates and breaks through the 

 thin* opposite side of the bud (stage 2) . The thick but pointed blade expands 

 in area and becomes thimier, and as it does so, it becomes apparent that there 

 are chamiels left open between the upper and lower surfaces; these are the 

 pre-pupal veins (stage 3). Soon the wing, as well as expandmg in area, be- 

 comes fatter by an inflation which forces the two surfaces apart, obhteratine^ 

 the venation until the wing is transformed into a featureless sac (stage ^,. 

 The greatest sweUing is reached just about the time of true pupation. 

 From then onwards, the wing starts to contract again. As the two surfaces 

 come together, they leave spaces between them: these are the adult vems 

 which appear first near the tip of the wing, and gradually spread back 

 towards its root (stages 5, 6, 7). At first the tissue between the vems is 

 spongy and loose in texture, but gradually it becomes more compact 

 the fluid which had filled the inflated wing sac being finally driven out 

 to give an immature wing ahnost identical in outline with the final adult 

 one, though smaller than it in size. In fact, after the last pupal stage drawn 

 in the figure nothing much happens to change the morpho ogy of the 

 wing except the expansion of the cells, throwing the whole structure 



Figure 15.1 



The centre colunm shows eight stages in the development o£ th.Drosophda 

 W during the pupal period, from the condition of an miagmal bud at the 

 Top'throTgh theVriod of inflation to the aduk condmon^On the eft are 

 thr e gene-controlled modifications affecting wmg shape: X.5f. produces a 

 distallick as early as the imagmal bud stage; broad affects the direction of 

 .rowth in the prepupal stage before the inflation ; Jmupy increases the longi- 

 S contraction after the mflation. On the right are three genes altermg 

 wing venation. Cubitus interruptus removes one of the P-P^P^l y-^-^^^^ 

 causi the appearance of extra venation, and vemlet the obliteration of the 

 dps of the normal veins, during the contraction followmg the mflation. 

 (Adult wings to smaUer scale; after Waddmgton mob.) 



