THE MUTATED GENE 225 



old may go only to the left or to the right. (Ludwig formulates 

 this situation thus: the cells right and left have both potencies of 

 development, and a gene-controlled reaction decides that the 

 higher quantity of a decisive substance appears on one side.) 

 In cases in which one side only is favored by heredity, there must 

 be present some additional mechanism comparable to the one in 

 molluscs which directs a flow or orientation only toward one side. 

 Other cases, containing at least some genetic information, are 

 as follows. Beliajeff (1931) finds in Drosophila that the mutant 

 abdomen rotatum, in the fourth chromosome, has always a 

 dextral effect, whereas the similar third chromosome mutant 

 rotated abdomen is always sinistral according to Bridges and 

 Morgan (1923). In rats, King (1931) found an inherited 

 unilateral microphthalmia with a tendency for development 

 on the right side. 



2. Another type of asymmetry is one in which an effect may 

 take place right or left or on both sides. This means that right 

 and left vary independently (no correlation) and that therefore 

 chance may produce the effect on neither side, on both sides, right 

 only, or left only. Such cases are characterized by a right-left 

 correlation coefficient nearing 0. For a number of Drosophila 

 mutations, such a condition has been described, e.g., by Guthrie 

 (1925) for eyeless, by Plunkett (1926) for achaete, by Astauroff 

 (1930) for tetraptera. The last author has made a special study 

 of the case of tetraptera. He concludes that here a variability 

 is involved which is more or less independant of the genie con- 

 stitution and of the environment but is caused by a local vari- 

 ability of the conditions of development. He works this out 

 in a very abstract way, but the facts could as well be described 

 by the action of an embryological process of the type that was 

 derived for the vestigial case. Astauroff thinks that the varia- 

 tions found in regard to the presence or absence of mirror sym- 

 metry in identical twins, as studied by numerous authors and 

 explained sometimes by somatic mutation in the case of absence 

 of symmetry (Newman, 1916), are also to be explained by 

 independent right-left variation. 



3. Within a genie effect produced on both sides of the body, a 

 certain more or less considerable variation occurs independently 

 on both sides of the body. The pattern is intrinsically symmetric 

 but in detail asymmetric. We mentioned this type for the differ- 



