340 PRINCIPLES OF EMBRYOLOGY 



There is a further important point about developmental paths, or se- 

 quences of epigenetic reactions, which has not been adequately illustrated 

 by the examples so far described. That is the existence of sharp distinc- 

 tions between the comparatively small number of alternative paths 

 which normally occur in the development of a particular species. In 

 embryological studies, we have seen (p. 179) how amphibian gastrula 

 ectoderm is competent to become either epidermis or neural tissue or 

 mesoderm, and in the great majority of cases becomes quite defmitely 

 either one or other of them. A similar situation is revealed by some genes 

 in Drosophila. For instance, Goldschmidt (1938, 1945) has shown that in 

 flies of certain abnormal genotypes, what would normally be the pre- 

 sumptive wing tissue enters on one of the alternative epigenetic paths, and 

 develops as a haltere; or in another genotype it becomes a leg. If the 

 normal aristopedia gene is replaced by its mutant allele {ss"), the tuft on 

 the antenna known as the arista may develop into the terminal section of 

 a leg, with perfectly developed claws. 



Genes which produce this type of effect, in which a part of the embryo 

 develops into an organ which is normally located somewhere else in the 

 body, are known as homeotic genes. A considerable number of them are 

 known in Drosophila. Besides aristopedia, for instance, prohoscipedia con- 

 verts the mouth parts into a leg-like organ; tetraltera converts the wings 

 into halteres, while tetraptera converts the halteres into wings; podoptera 

 causes the wings to become legs, while hithorax changes the whole meta- 

 thorax into a mesothorax. Some of these effects can also be produced by 

 environmental agencies; thus Gloor (1947) found that ether treatment of 

 the young larva a few hours after laying would produce a phenocopy of 

 hithorax, presumably by changing the condition of the larval hypodermis 

 at the time when the imaginal buds are first being formed. Similar changes 

 can, however, also be produced at a much later stage since, as we saw 

 (p. 141), abnormalities in the folding of the imaginal buds just before 

 pupation may cause parts of them to develop into tissues which should 

 belong to another part of the body. It is probable then that most of the 

 homeotic genes act in the first place at the time imaginal buds are first 

 formed but that these alterations do not produce any defmitive effect 

 until about the time of pupation. 



Once a piece of tissue has entered on a path of development, its final 

 condition can be affected by all the genes which are concerned in that 

 path. Thus, if, in an animal in wliich the arista takes the leg path owing to 

 the presence of the mutant 55" gene, the genes concerned with the develop- 

 ment of the leg have been replaced by mutants such as fourjointed wliich 

 produce shortened legs, then tlie aristal leg will also be shortened; while 



