98 



EVOLUTION AND GENETICS 



GROUP IT 



The hereditary elements of Group IV are carried 

 by the pair of very small chromosomes. Only three 

 mutants have been obtained. One of these, called 

 "eyeless" (fig. 43 a, a"), is variable — the eyes are 

 often entirely absent or represented by a few or by 

 several ommatidia (h, h') . On the sides of the head 



'^j 



1-* 



a 



a 



w 





Fig. 43. — The three characters of the foiirth chromo- 

 some of Drosophila melanogaster. See text. 



where the normal eve lies there is, in "eyeless," a 

 corresponding empty area in the more extreme con- 

 dition {a, a^) , and even when a piece of the eye is 

 present it lies in this area but failing to fill all of it, 

 the outline of the full sized eye is, so to speak, still 

 present. These parts of eyes {b, h') might be spoken 

 of as rudimentary organs. 



