18 J. K. BREITENBECHER. 



If these mosaics of Bmchus had originated from heterozygous 

 cultures, then the elimination of one autosome might produce one 

 elytrum of one color and the other of a different one; but since 

 these mosaics came from homozygous cultures, it is evident that 

 the elimination of an autosome previous to or even after fertiliza- 

 tion could not have this effect. It is difficult to conceive of any 

 kind of autosomal non-disjunction that could possibly create a red 

 elytrum, for example, when the insect was pure for black. Neither 

 can non-disjunction nor the elimination of an autosome have any 

 effect in causing this kind of a mosaic unless one assumes that a 

 mutation occurs after fertilization. The evidence is in favor of 

 this interpretation because these mosaics are not transmitted. It 

 appears, therefore, more reasonable to assume that a mutation has 

 occurred in one autosome during the embryonic development of the 

 insect. Neither is it necessary to assume that autosome elimination 

 or non-disjunction is essential to account for these mosaics even 

 after a somatic mutation has taken place, unless these mosaics were 

 recessives instead of dominants. 



Normally recessive mosaics could not be seen, even though they 

 occurred frequently, because a recessive autosome mutation could 

 not be visible in the F progeny, except through autosome elimina- 

 tion. To illustrate the appearance of such a mosaic, let us suppose 

 in a pure culture for red that a female was found with one black 

 elytrum (black is recessive to red). The normal autosome com- 

 plex on each side of the body of a homozygous red female would 

 be R R. If a mutation to black would occur in one autosome on 

 her right side (R b is the formula for black), then her right side 

 would be R R b , and her left side, normal, R R ; but because red is 

 dominant to black, when heterozygous, she would still appear red 

 unless the autosome carrying R (red) on her right side would be 

 eliminated, then one side would be black and the other red. 



The time in the ontogeny at which the mutation occurred would 

 eovern the extent of its effects. If it took place at the first cleav- 



o * 



ages, for instance, the entire half of the body might be affected; 

 but if it took place later at the time when the anlagen of the wings 

 are differentiated, then one elytrum would be black and the other 

 red. 



It is true, however, for every mosaic so far discovered in 



