2O J. K. BREITENBECHER. 



Drosophila," to quote these authors, " makes it highly probable that 

 when a mutation occurs it takes place in only one chromosome of 

 the pair. Hence any mutation in somatic tissue, if recessive, would 

 be concealed by the presence of the normal allelomorph in the 

 homologous chromosome." This is the key to the situation for 

 Bruchus, since these thirty-one mosaics appear through dominant 

 somatic mutations in one chromosome of the pair. 



It is evident that these somatic mutations in Bruchus concern the 

 autosome in which the multiple allelomorph gene for R (red) is 

 located. The difference between the factor mutations which the 

 author ('21) discovered for the body and elytral color factors 

 located at the locus, R, in this autosome and its somatic mutations 

 as manifested in these elytral mosaics is that the former are trans- 

 mitted, while the latter are not. 



The evidence further indicates that there is a chromosome con- 

 tinuity between the gene for R (red) in this autosome in the germ 

 cell and this same gene as manifested through the thirty-one mosaics 

 in Bruchus. Of these somatic mutations, twenty-two occurred in 

 homozygous black cultures, through a mutation from recessive 

 black to dominant red ; only one from a pure white culture, through 

 a mutation from recessive white to dominant black; while eight 

 appeared from wild stock, seven of these mutated from recessive 

 tan to dominant black, and one of these from recessive tan to 

 dominant white. 



The most noteworthy result is that germinal mutations and 

 somatic mutations are identical factor mutations, because both 

 originate through a mutation in a chromosome. This paper fur- 

 nishes evidence in favor of the chromosome hypothesis. 



The conditions noted in these unilateral somatic mosaics perhaps 

 suggest that the chromosome mechanism may also be found to 

 account more generally for the bilateral type of body symmetry, at 

 least in certain groups. Conklin has pointed out that while types 

 of cleavage, symmetry, etc., are determined by cytoplasmic influ- 

 ences, this is not at variance with the chromosome doctrine, since 

 the cytoplasm is itself influenced by the nucleus through the large 

 amount of nuclear material which escapes at every mitosis. The 

 investigations regarding cytoplasmic localization and their part in 

 inheritance merely shows " that in early development inherited 



