136 ANIMAL LIFE AND SOCIAL GROWTH 



sort of an axial organization. In its simplest 

 terms the main axis of an animal is that which 

 runs from the head through the tail. Other axes 

 may be present which are secondary to this 

 primary axis. According to modern theories of 

 heredity, each cell of a plant or animal receives 

 similar chromosomes bearing similar genes, as 

 the bearers of hereditary qualities are called. 

 According to these theories, each cell from one 

 end to the other contains the same sort of heredi- 

 tary material. Something must work on this 

 common heredity to cause one end to develop into 

 a head and another into a tail. 



In the marine plant, Fucus, one of the large 

 marine algae, the determination of the axis is 

 definitely under the control of environmental 

 forces. Ordinarily when the eggs of this plant 

 are uncrowded, the axis is determined by exposure 

 to light and the end which is exposed to the action 

 of light becomes the most rapidly growing, apical 

 end. If, however, Fucus eggs are in darkness and 

 are somewhat crowded together, that is if they 

 are within 0.02 mm. of each other, then the part 

 of the egg most nearly free, forms the apical end. 

 Under these conditions the position of the egg 

 with regard to its nearby neighbors determines 

 which end will become free and which will be 

 attached. 



