GENES AND DEVELOPMENT I43 



the fundamental features of the organism which depend on these 

 cytoplasmic anangements are transmitted cytoplasmically and not 

 through the nucleus. 



Work on another organism with mosaic eggs, the snail Limnea,i has 

 raised the possibility that although the fundamental plan of the body 

 is dependent on the cytoplasmic arrangement of the egg, that arrange- 

 ment itself may be controlled by the genes of the mother. Two forms 

 of the snail are known, one of which is coiled in a right-handed, the 

 other in a left-handed spiral. The way in which any given snail coils is 

 determined by the cytoplasm of the egg from which it developed ; but 

 the cytoplasm of the egg is dependent on the genetic constitution of the 



Fig. 69. The Inheritance of Coiling in Limnea. — The gene D for right-handed 

 (dextral) coiling is dominant to d for left-handed (sinistral) coiling, but the 

 direction in which a snail coils is determined not by the genes it contains but 

 by those its mother contained, e.g. 



dextral DD x sinistral dd 



F1. dextral Dd 



I 

 (selfed) 



F2. AH dextral, with genotypes 1DD : 2Dd : ^dd 



I I I 



(selfed) (selfed) (selfed) 



F3. dextral dextral sinistral 



mother in which the egg was formed, right-handedness being domi- 

 nant. Thus from the evolutionary point of view, where we are interested 

 not in single ontogenies but in whole series of them, we can say that 

 the direction of coiling is inherited by a nuclear, genetic mechanism. 

 The same may perhaps be true for the organ-forming substances of, 

 for instance, the Ascidian egg. 



4. Insects 



The embryology of insects^ is of particular interest to geneticists 

 because that group has provided, in Drosophila, the best material yet 

 discovered for genetical investigation. Unfortunately it is not equally 

 good as a subject of experimental embryological research. 



The early investigations of insect eggs showed them to be highly 



^ Boycott and Diver 1923, 1930, Sturtevant 1923. 



2 Revs. Bodenstein 1936, Richards and Miller 1937, Seidel 1936. 



