l84 AN INTRODUCTION TO MODERN GENETICS 



"frozen" by the occurrence of metamorphosis. It is found that there is 

 only a certain period during development (just before the facets actually 

 appear) during which temperature changes affect the number of facets 

 formed in Bar flies. The interpretation suggested is that during early 

 development a facet-forming "substance" is formed and that the Bar 

 gene sets going reactions which break down this substance; then a 

 third set of processes determine that facet formation shall actually 

 begin^ and a number of facets are formed proportional to the amount 

 of substance still present. The temperature effects, which only occur 

 when the Bar gene is present, presumably affect the break-down pro- 

 cess for which' Bar is responsible. In this example the end-product 

 which is the "effect" in the time-effect curve is not a single substance 

 like a pigment, but is a relatively complicated tissue, the eye facets. 

 The facet-forming substance, however, may be a single chemical com- 

 pound, since Ephrussi^ has recently shown that the number of facets 

 formed in Bar eyes may be increased by the injection of suitable 

 extracts from normal pupae. We know nothing about the mechanism 

 of action of the substance ; it might either increase some sort of induc- 

 tive stimulus to facet formation or lower the threshold of reaction to 

 such a stimulus. The substance is probably related in some way to the 

 vermilion substance, and may be identical with it; it is known that Bar 

 inhibits the formation of vermilion substance in the eye itself, though 

 not in the rest of the body. 



In considering development from an embryological point of view 

 we can, as with Bar, not yet express the characters in which we are 

 interested in terms of quantities of definite substances, but must talk 

 instead of histological types such as neural tissue, eye facets, etc. But 

 experimental embryology leads to the formulation of exactly the same 

 kind of system of alternative possibilities as we have had to develop to 

 describe the genetical results. For instance, the ectoderm of the amphi- 

 bian gastrula has two alternative methods of change open to it; it may 

 become epidermis, or, if the evocator is added to it, it may become 

 neural tissue. The case is exactly parallel to that of the pigment system 

 in Drosophila at one of its branch points. Both the methods of 

 approach to the study of development formulate the main problems in 

 the same kind of way, and we may hope that genetics and embryology 

 can collaborate in finding the answers. 



We must now consider certain genetical problems in the light of 

 the scheme of thought which has just been developed. 

 1 Ephrussi, Khouvine and Chevais 1938. 



