CHAPTER 2 

 PRIMARY ORGANIZATION OF LIVING MATTER 



No feature of organisms has so many and such varied consequences 

 as the fact that they are composed of protoplasm which is usually 

 arranged in the form of cells. If a bit of animal tissue, cut thin, be 

 it from muscle, gland, skin, brain, or sense organ, is examined under 

 a microscope, it is found to be blocked off in small areas, all of which 

 resemble one another in certain respects and some of which ai-e alike in a 

 great many ways. These are the cells. We have seen (pages 14, 15) how 

 the existence of cells gradually became known, and how much this dis- 

 covery influenced work in different fields of biology. The authors of the 

 •ceil theory, as it was first formulated, were content to claim that all things 

 are composed of these units. Its immediate effect was therefore only on 

 the structural side of biology, as has already been related. Had the 

 theory developed no further, it would have continued to affect only 

 morphology. When, however, the chemical and physical composition of 

 the protoplasm was studied, and when the minute structure of the parts 

 of the cells began to yield to the microscope, it became apparent that the 

 existence of cells was highly important in physiology, heredity, and 

 evolution. A knowledge of cells therefore lays a foundation for much of 

 the rest of biology. 



The Size of Cells. — It is surprising to find how much difference there 

 is among cells with respect to size. The radius within which the various 

 activities of cells must occur should be of some significance. Each cell 

 consists typically of a nucleus lying within a bit of protoplasm which is 

 the cell body or cytosome. Important reactions take place between 

 the different parts of the cell. Since the nearness of these parts to one 

 another must influence the ease with which they work together, the 

 size of the cell should be of some importance. Yet cells show very great 

 differences in this respect. Some bacteria are so small as to be almost 

 invisible even with a good microscope; somewhat larger are most tissue 

 cells, which are quite easily seen when thus magnified but cannot be seen 

 without such aid; but all these are topped by the egg yolks of the larger 

 birds, which are 2 or 3 inches in diameter. Nerve cells often have great 

 length, particularly those which extend from the spinal cord to the ends of 

 the extremities in man or the other large mammals, but are quite slender. 

 Sometimes these great differences in size fit the cells for their particular 



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