34 PRINCIPLES OF ANIMAL BIOLOGY 



Polarity. — The structures described above are the chief features 

 in which different parts of a cell are visibly distinguishable from one 

 another. They do not, of course, constitute all of the organization which 

 cells are known to possess; and other elements of organization are de- 

 scribed below. But along with other general features is to be included 

 an invisible organization which is termed polarity. In many cells, one 

 portion is destined to perform certain functions, another portion certain 

 other functions, even when these portions are visibly alike. In a develop- 

 ing egg, one part will become the nervous system and associated sense 

 organs, another part the digestive tract. In the ordinary course of 

 development these parts are not interchangeable. This evident arrange- 

 ment of parts as evidenced by their future activities, is the phenomenon 

 which is called polarity. At either end of an axis the protoplasm is 

 different. 



A good example of polarity is found in the eggs of insects. One 

 end of the egg, in some way different from the other end, always becomes 

 the head. One side always is right, the other left. The polarity goes 

 back into the ovary (reproductive body) for, at least in many insects, 

 the same end of the egg always emerges first when the egg is laid. 



Occasionally this polarity is recognizable in advance because the 

 parts are differently colored; or one part may contain oil droplets, or 

 some other cell inclusion, another part none. In many cases it has been 

 definitely proven that these visible differences are not the cause of polarity, 

 they are merely associated with it, or are signs of it. In other cases 

 where proof is wanting that the visible bodies are not causal agents, it 

 may still be doubted whether they are more than incidental phenomena. 

 The polarity appears to be an invisible organization of the protoplasm 

 itself. 



Structural Relation to Other Cells, — When cells are free-living and 

 independent, as in the Protozoa, they may have little or no influence 

 upon one another. When they are aggregated into masses, as in the 

 multicellular animals, there is always the possibihty that each cell may 

 be modified, and its activities guided, by the cells around it. Often 

 such interdependence must follow merely from the diffusion of fluids 

 from cell to cell, or from surface phenomena. 



In some cases, however, protoplasmic connections are known to ex- 

 tend from one cell to another. These have been demonstrated in the 

 skin of the salamander (Fig. 21), are conspicuous in Volvox, and have 

 been described for many kinds of animal cells. In plants, it is maintained 

 that cell-bridges are usuall}^ present, the fine protoplasmic filaments 

 passing through minute pores in the cell walls. This connection be- 

 tween cells opens up large possibilities of mutual influence. Some of the 

 consequences of it are pointed out later. 



