io INDIVIDUALITY IN ORGANISMS 



very often the chief direction of growth, so that the body 

 becomes elongated in this direction and the polar axis 

 becomes the longitudinal axis. In short, the so-called 

 polarity of the individual represents the direction of the 

 chief or major order, while the axes of symmetry repre- 

 sent the directions of minor orders. 



The two terminal regions of the polar axis exhibit in 

 general distinct and characteristic differences in behavior 

 and structure. In most plants, in sessile animals, and 

 in radially symmetrical forms generally, these two 

 regions are commonly called the apical and basal regions, 

 while in bilaterally symmetrical motile animals they 

 are usually known as anterior and posterior. The apical 

 or anterior region is primarily the region of greatest 

 dynamic or metabolic activity in the individual: in the 

 plants it becomes the growing tip, the region of most 

 active primary growth, while in the animals it becomes 

 the most highly specialized and differentiated region of 

 the body, and in those forms which possess a central 

 nervous system, including all except the simplest animals, 

 the chief part of the central nervous system, the cephalic 

 ganglion or brain and the chief sense organs usually arise 

 in this region; in other words it becomes the head and 

 in motile forms usually precedes in locomotion. 



The basal or posterior end, on the other hand, is 

 primarily the least active region, although in many 

 forms it may become secondarily a region of growth or 

 specialized activity because of certain changes during 

 the life of the individual which will be considered later. 

 In sessile forms it is usually the region of attachment and 

 may develop special organs of attachment, while in 

 motile forms its activity is more or less under the control 



