28 PRINCIPLES OF ANIMAL BIOLOGY 



nucleus and in some cases characteristically near the centrosphere. The 

 function of the Golgi apparatus is still unknown, though there is some 

 indication that it takes part in the process of secretion by gland cells. 



Besides all the above structures which serve, or ma}^ serve, some func- 

 tion in the cell, and which may therefore be regarded as cell organs, there 

 are often lifeless matters enclosed in the protoplasm. These may be 

 grains of starch, or oil or fat globules, which the cell has produced and 

 which are stored as future food. Or the lifeless objects may be undigested 

 remains of organisms taken as food, or even objects picked up incidentalh' 

 along with food or otherwise. These nonliving objects may be spoken of 

 as cell inclusions. 



Polarity. — Beside the differentiations described above, cells may 

 possess another type of organization which is termed polarity. One por- 

 tion is destined to perform certain functions, another portion othei- 

 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 shown l)y their future activities, is the phenomenon 

 which is called polarity. Examples of polarity are found in the eggs of 

 insects, in which one end of the egg, in some way different from the other 

 end, always becomes the head. Other cells than eggs are commonly 

 polarized. Thus, cells bearing cilia (hairlike projections) on one end are 

 polarized. So also is the connection (synapse) between nerve cells, since 

 nerve impulses travel over it in only one direction. Many gland cells 

 receive materials from the blood on one side and after working them o^-er 

 extrude the product into a chamber on the opposite side. When long 

 slender cells standing on end are crowded together to form a layer covering 

 the surface of some organ, the nuclei of the cells are usually near the lower 

 end. These are all polarized cells. In some cases the polarity is visible; 

 but, before the structures indicating the polai-ity were developed, there 

 was presumably an invisible difference in the proto])lasm. The nature 

 of this organization is not known, and there is much disagreement as to 

 whether it is inherent in the cells or is impressed on them by external 

 circumstances. 



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 possibility 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 electric phenomena. In some cases, however, proto- 

 plasmic connections extend from one cell to another. These have been 



