THE GAMETES 43 



arrangement is brought about. In particular, what is the relation between 

 it and the hereditary factors or genes which determine the detailed charac- 

 ter of the adult organism ? It was for a long time argued by many biolo- 

 gists that genes affect only minor and trivial processes in the latter stages 

 of development, while the major outlines of the animal body are deter- 

 mined not by them, but by the nature and structure of the egg cytoplasm. 

 This theory made no suggestion as to what determined the nature of the 

 egg in its turn, and was therefore always somewhat incomplete. But this 

 was not its worst fault; in the only suitable cases which have been pro- 

 perly investigated, we have evidence that the theory is incorrect. 



There are not many examples of animals in which there are two or more 

 variants in the basic structure of the egg cytoplasm, but a few cases are 

 knoMOi. For instance, the eggs of molluscs cleave with a spiral pattern 

 (p. 60) ; and this spiral may be either right- or left-handed, a few species 

 having variants of the two kinds. In the pond-snail Limnea, the inheritance 

 of such variations was studied by Boycott and Diver (1923, 1930). The 

 right-handedness or left-handedness of an egg depended in a straight- 

 forward way on the genes in the mother in whose ovary it was formed. 

 This is a clear case of a difference in egg cytoplasm, which is inherited by 

 means of genes, exactly as are differences, for instance, in the formation of 

 pigment in an eye-cell. The only slightly odd feature of the situation is 

 that, whereas one normally looks for pigment within the cell in question, 

 we fmd it more convenient to diagnose the nature o£ Limnea egg cyto- 

 plasm by waiting to see which way the fertihsed egg will cleave; but 

 that is a mere matter of the technique of study. The important point is 

 that in this case we can prove that the fundamental outline of the embry- 

 onic organism is based on the egg cytoplasm, but that this in its turn is 

 determined by genes, just as any other character is (Fig. 2.8). It is still quite 

 uncertain how these genes operate. One possibility is that they influence 

 some asymmetry which might exist in the protein molecules of the egg 

 cytoplasm ; but this is rendered unlikely, though not perhaps impossible, 

 by the fact that the structure of the sperm, which are also spirahsed, is not 

 affected by the genes which control the symmetry of the eggs (Selman 

 andWaddington 1952). 



Other examples of the control of the egg cytoplasm by the genes in the 

 maternal ovary are provided by the 'female-steriles' in Drosophila (p. 135). 



We may almost certainly conclude that in all eggs the basic structure of 

 the cytoplasm is laid down in the maternal ovary while the ovum is 

 being formed, and that it is as fully dependent on the genes of the mother 

 as is the character of her eyes or skin or hair. If this is so, we should expect 

 to fmd a regular relationship between the main structure of the egg and 



