Genie Control of Development 413 



(tetraltera) loci consists in disturbing the common differentiation of 

 the four anlagen (centers, fields, territories, in the language of experi- 

 mental embryology) by forcing them to differentiate separately, with 

 strange consequences. The amount of the final effect, visible as pene- 

 trance and expressivity, varies through a series of grades. Clearly there 

 is, as in the former example, a period of labile determination; the 

 earlier the pod and tet action produces the separation of the four 

 embryonic fields, the more extreme are the transformations of the 

 wing. We consider these now individually. 



Fig. 21. Series of thorax structure of mutant tetraltera in D. melanogaster, 

 including scutellum and wings showing changes of determination in these organs. 

 Semidiagrammatic. (From Goldschmidt, 1952fo.) 



The anterior anlage, the cubital one, may be separated alone 

 (figs. 21, 22); the rest of the wing may then form more or less normal 

 wing tissue. The cubital part, however, transforms into a trisegmented 

 leglike structure, the wing leg of podoptera. The way in which this 

 happens introduces a new feature into our discussion of genie action. 

 At the "raw edge," where the anlage is separated from the posterior 

 end, a mirror-image half of the cubital part with its two rows of 

 marginal hair is formed, thus transforming the cubital part into a 

 symmetrical whole, which assumes the leglike structure. This means, 



