148 



OTTO I.. MOIIU 



the singed from the wild-type zone follows the median plane, except 

 on the dorsal side of the thorax, where the singed region proceeds 

 over the median line, including the upper half of the right lateral 

 surface of the thorax. From here the singed zone also continues 

 along the collum to the head, where a narrow ventro-lateral part on 

 till' rigiit side is singed. The rest of the individual has normal wild- 

 type hairs and bristles. 



III. THEORETICAL CONSIDERATIONS. 



The striking somatic peculiarities mentioned made it at once prac- 

 tically certain that we were dealing with a sex-linked recessive mu- 

 tation, which had occurred in one of the very early cleavage nuclei 



Fig. 3. The mosaic as seen from the right side, 



and which caused the bristle and hair alteration in the singed part 

 of the mosaic. 



The individual could not be interpreted as a gynandromorph. 

 On this assumption the fly would have started as a two X individual, 

 a female, heterozygous for a recessive sex-linked mutant gene, which 

 produced the bristle and hair alteration described. If elimination of 

 one of the wild-type daughter .Y's occurred in one of the very early 

 cleavage cells, then one of the daughter cells would obtain only a 

 single X, viz., the one which contained the mutant gene. The part 

 of the individual which was derived from this cell would accordingly 

 be male, (XO), and show the corresponding mutant character. The 

 rest of the individual would be female and wild-type. Any such 

 gynandromorph explanation had of course to be abandoned as soon 



