80 SPIRAL TMIST AXD OPT. AVTlYrVY 



hereafter, call these two kinds the typical and the in- 

 verse individuals.) 



It may be supposed that the direction of the spiral is 

 determined by some dissymmetric substance which is 

 labile, in the sense that it can undergo an inversion of 

 its molecular configuration with comparative ease, and 

 that by means of such a mutation, the form of the or- 

 ganism can change. Such an idea is due to Koltzoff 

 (1934) and to Needham (1934), according to whom the 

 origin of dextrality and sinistrality, as observed in the 

 eggs of certain snails and later in their body, is connected 

 with the stereo-chemical properties of some of their com- 

 ponent protein molecules. But, at present, these rela- 

 tions are very obscure. 



Koltzoff expresses himself as follows : ''Particularly in- 

 teresting is the case when in a pond-snail, Limnaea ru- 

 hella, in one and the same species genotypes are found 

 which are characterized by either the left or the right 

 spiral types of shell. The cleavage of the ovum in those 

 genotypes proceeds, correspondingly, according to the 

 right or left spiral types. Here, already at the first 

 division of the ovum, the difference between both types 

 is marked in a sufficiently distinct manner by the posi- 

 tion of spindles. It is very probable that the right and 

 the left types are distinct already in the unfertilised 

 ovum, because they can be detected in the relative posi- 

 tion of both directing bodies. Hybridological analysis 

 shows that this character is determined by one pair of 

 allelomorphs. The right twist of the spiral is determined 

 by the dominant gene, the left twist by the recessive one. 

 The mother homozygous as to the recessive gene can 

 itself have the right spiral (because the ovocyte from 

 which it evolved could be heterozygous), but all its eggs 

 develop according to the left type, even if they were 

 fertilized by the sperm of the homozygous dextral fa- 

 ther. This shows that genotypical peculiarities of the 

 male nucleus are not manifest on this stage. On the 



