224 PHYSIOLOGICAL GENETICS 



Drosophila mutants mosl colorations are symmetrica] and 

 changes of structure of organs asymmetrical. He thus comes 

 near to finding the explanation which was given above for the 



vestigial case ami which is also to be found more or less vaguely 

 expressed in other work on the problem. 



A similar case occurs also in the work of Boycott et al. (1930) 

 on Lymnaeus to which we shall return immediately and which 

 belongs otherwise to a different type. Here a left-handed line of 

 shells exists, differing by a mutant gene from the normal right- 

 handed one. In these lines, right-handed forms also occur which 

 are not hereditary, although their percentage of occurrence is 

 hereditary. Whether this is caused by modifiers or by different 

 alleles for left-handedness, in some way an embryonic (here 

 cellular) situation near a threshold line must be involved, which 

 might be transmitted or not by some individuals according to 

 genetic conditions controlling a process involved in the threshold. 

 If it should be proved that such lines are actually based upon 

 multiple alleles, the interpretation would have to be parallel to 

 the one for the vestigial case. 



We started our discussion of the problem of asymmetry with a 

 case that seemed to be instructive because the asymmetry con- 

 sisted in existence or nonexistence of the effect on both sides only 

 in the first steps of a quantitative series. However, the most 

 frequently studied types of asymmetry are different in various 

 aspects. We shall now consider three such types. 



1. Constant asymmetry of an alternative type. For example, 

 a shell is hereditarily coiled either in a left or in a right-handed 

 spiral. (For examples of this and other types and their details, 

 see Ludwig's monograph and his recent review, 1936.) Here we 

 have the well-analyzed case of Limnaeus (Boycott and Diver, 

 Boycott et al.) in which a mutant gene changes the direction of 

 coiling (see page 206), and parts of which we have already 

 considered. Occasionally, however, a nonheritable variation 

 to the other type occurs (Lang, 1908; Boycott et al.), the fre- 

 quency of which might be hereditary (see p. 206). 



In this case, the cytological basis of the asymmetry is known 

 (see page 206). In other similar cases, we must assume a parallel 

 process, i.e., the presence of a strictly alternative situation at 

 some point of embryonic determination where a determining 

 material for the formation of an organ or for controlling a thresh- 



