38 



GENERAL ZOOLOGY 



greater part of the brain substance is made up of nerve 

 fibers, other types of cells, and- their products. The whole 

 body contains a vast number of such units. Though cells 

 are highly variable in size and shape, they have certain 

 structures which are familiar to everyone who has looked 

 through a microscope at any thing living. 



If you scrape a little of the membrane from the inside of 

 your cheek and place it under a microscope, the fragments 

 will appear like Fig. 22. The individual cells are flattened 

 and irregularly hexagonal in form. Each one has, at the 

 center, a body of somewhat denser material, the nucleus. 



FIG. 23. Examples of cells. A, cartilage; c, cells; s, the substance between 

 the cells. B, a muscle cell from a round- worm; m, muscular or contractile 

 portion; n, nucleus. C, nerve cell; a, axis cylinder process; d, dendrites, or root- 

 like projections; n, nucleus. 



The rest of the cell substance, i.e., the outside part which 

 surrounds the nucleus, is the cytoplasm. Other parts of 

 the body will show cells somewhat like those from the 

 cheek. All have a nucleus and cytoplasm, but there may 

 be great variation in both. Some cells have walls of 

 secreted material around them which forms " cell-walls," 

 others have the cytoplasm drawn out into long projections, 

 or vary in other ways. Fig. 23 shows a few examples of 

 cells. 



Fig. 24 gives a general scheme of a typical cell with all 



