336 



PRINCIPLES OF EMBRYOLOGY 



N 



M 



Figure 15.3 



The genetic control of bristle development in Drosophila. A and B are sec- 

 tions through the thoracic hypodermis of the mid-pupa, showing two 

 stages in the development of the bristle-forming (trichogen, trch) and socket- 

 forming (tormogen, tor) cells. C, surface view. D, surface view showing the 

 extra division which occurs in split, giving (£) duplicated bristles. F, surface 

 view, and G, section, in Hairless, when the trichogen and tormogen lie side 

 by side at the same level, and give two sockets (H). In shauen-nakcd {I, J,) 

 the situation is similar but usually not so extreme. In Stubble {K, L, M) the 

 tormogen does not embrace the trichogen as closely as usual, and an ab- 

 normal thick bristle is produced. N shows the irregular bristles produced by 

 ^orked. (From Lees and Waddington 1942.) 



former and three of the latter. Another gene, dichaete, has a somewhat 

 similar action, but it is usually only the tormogen which divides a second 

 time and becomes doubled, hi the next step, the precise arrangement of the 

 tormogen above and to one side of the trichogen is also under genetic 



