72 



THE AMERICAN NATURALIST [Vol.LVI 



plants. Among animals, Drosophila (Morgan and 

 Bridges, 1919) has furnished several examples of un- 

 doubted somatic mutation resulting in mosaic individuals 

 other than gynandromorphs. 



Somatic Segeegation 

 Bud variations have probably been ascribed to somatic 

 segregation more frequently than to any one other cause. 

 Perhaps the opinion commonly held that bud variations 

 occur more frequently in hybrids than in other material 

 and the long known fact that seed-grown offspring of 

 hybrids exhibit segregation, is chiefly responsible for this 

 usage. It is, of course, possible that most vegetative 

 variations are of this nature, but the fact that the indi- 

 viduals in which they arise are frequently found to be 

 heterozygous for the genes concerned is no conclusive 

 evidence that segregation is involved. Mutations also, 

 as noted by several writers, are most likely to appear in 

 heterozygous material because most of them are recessive 

 and the unmutated dominant allelomorphs prevent their 

 expression in the individuals in which they originate if 

 the latter are homozygous. 



Chromosome Elimination.— The best examples of so- 

 matic segregation that have been subjected to critical 

 genetic analysis are afforded by the work with Droso- 

 phila. It has been shown by Morgan and Bridges (1919) 

 that, of the relatively numerous gynandromorphs which 

 have appeared in the course of investigations with Dro- 

 sophila, nearly all have resulted from the elimination of 

 the sex chromosome at some early cleavage division. If 

 a fertilized egg starts as a female, XX, and one X chro- 

 mosome is eliminated at an early segmentation that part 

 of the individual developing from the cell that receives 

 but one X chromosome should be male, X( ), wliile the re- 

 maining part should be female, X X. 



The evidence in support of this \ icw w.is ohtniinMl t'loni 

 crosses the parents of which had di iVcivnt sex liiikrd ;md 

 different autosomal characters, tiiat is, cli.i l actiM s whose 

 genes are carried by the sex cliromosoiiifs and 1)\ ihe 

 autosomes, respectively. The male, as well ;i> the female, 



