298 



EMBRYOLOGY 



FUCUS EGG 



V T^NUCLEUS 



LIGHT 



PRESUMPTIVE 

 THALLUS CELL 



PRESUMPTIVE 

 RHIZOID CELL 



Fig. 191. Determination of polarity in the egg of Fucus by means of light. 

 Left: The fertilized egg develops into two cells, one of which is a presumptive 

 thallus cell and the other a presumptive rhizoid cell. There are no external 

 manifestations of polarity in the egg. Right: A beam of light is directed against 

 one surface of the egg. As a result the presumptive rhizoid cell always forms 

 opposite the side treated with light. Chemical substances will also determine 

 where the rhizoid cell forms. 



just as experimentally it is possible to subject the egg of Fucus to different 

 chemical environments on the two sides and so determine its initial polarity. 

 In this way if we assume an interaction of the same genes with different 

 cytoplasms we obtain a formal explanation of development. We could, for 

 example, consider a group of genes as affecting trunk structures as opposed 

 to head structures. 



Often a gene which affects chiefly one structure may also affect a number 

 of other structures in the body. Such a phenomenon could be understood if 

 we were to assume that the secondary effects of the gene were due simply to 

 the fact that the chemical composition of the various structures affected is 

 similar. The multiple effects of a gene result in a syndrome, and it involves 

 a number of abnormalities linked together. The abnormalities need not be in 



