344 PRINCIPLES OF EMBRYOLOGY 



degeneration of the cells of particular tissues, or, when the effect is weaker, 

 by a retardation of growth. This pattern differs in detail in different 

 stocks of the lethal, depending on the other genes associated with it (the 

 'genetic background'). More important from the present point of view 

 is the fact that the pattern in which the gene normally manifests itself is a 

 mixture of primary and secondary effects. That is to say, in some tissues 

 the abnormality is a direct consequence of the activity of the lethal gene in 

 those particular cells (a primary effect), while in other tissues the gene may 

 either be inactive, or ineffective because its action fails to surpass some 

 threshold, yet the tissue may develop abnormally because it is influenced 

 by unusual substances produced elsewhere in the body (a secondary 

 effect). Hadom demonstrated the reahty of such secondary effects by 

 showing that certain organs from larvae of a lethal type will continue 

 to develop more or less normally if transplanted into host larvae of a 

 wild-type strain. For instance, lethal-meander and lethal-translucida both 

 die usually at about the time of puparium formation; but the imaginal 

 buds, if transplanted into normal hosts, can carry through a complete 

 metamorphosis. Their death when left undisturbed is therefore a second- 

 ary consequence of abnormalities in the development of other parts of 

 the body (probably the protein-metabolising system in the gut of lethal- 

 meander and some other nutritive or hormonal peculiarity in lethal- 

 translucida). 



The distinction between primary and secondary effects can of course 

 also be made within the confmes of a single organ. For instance, if the 

 primary effect of a gene is to cause the absence of large parts of the 

 anterior and posterior regions of the wing (e.g. Beadcx or Lyra) a second- 

 ary consequence is that when the pupal contraction occurs the longi- 

 tudinal veins become squeezed nearer together and diverge at a smaller 

 angle (Waddington 19406).) The fact that in Hadom's cases the secondary 

 effects occur in different organs to the primary ones is not the essential 

 point of the distinction, but merely makes it easier to recognise which 

 effect is nearer the very first influence of the gene on the sequence of 

 developmental processes. 



Hadom has provided a diagram, reproduced in Fig. 15.7, which ex- 

 presses neatly some of the ways in which secondary effects may occur. 

 In this, the rectangles i, 11 and m represent three cells in three different 

 organs, as it might be Imaginal Bud, Fat Body and an Endocrine Gland. 

 In each cell ten genes are represented. It is supposed that in each type of 

 cell the heavily ringed genes are active, producing substances (large rings) 

 which fit together into a reaction-chain. Consider what would happen if 

 various of these genes mutated. If the activity of 8 was altered, nothing 



