CHAPTER III. 

 THE ANIMAL CELL; ITS MORPHOLOGY AND PHYSIOLOGY. 



30. Introduction. In studying the structure of organisms 

 two methods are open to the student of to-day. He may 

 begin with the whole adult individual and by dissection he may 

 reach a knowledge of the constituent parts, organs, tissues, 

 cells. This, the analytic method, is the method of history and 

 has given us the mass of details which we have at present. 

 On the other hand, it is possible to avail one's self of the 

 results of such studies, to assume the unit of structure which 

 is uniformly found, and, by a synthetic process, follow the 

 building up of an organism' from its elementary parts. This 

 is the process which the development of the individual illus- 

 trates. It has the special advantage of emphasizing the funda- 

 mental unity of origin of the organs, and the likenesses of 

 organisms, and gives the true significance of differentiation 

 and development. 



31. The Cell. Having discussed in Chapter II the sub- 

 stance in connection with which life manifests itself, it is 

 necessary to recall the fact that the protoplasm of an organism, 

 while connected in various ways, is separated by boundaries 



^ into unit-masses, each mass having the essential qualities of 

 the whole. Each unit mass of protoplasm is called a cell. The 

 cell is not to be considered as the ultimate unit of structure; 

 it is itself a group of bodies which are in turn composite. It 

 is thus to be looked upon as an organised structure. 



/32. Cell Form. Cells, unhampered in the direction of 

 growth, tend to assume a spherical form. Agencies both in- 

 ternal and external, as nutritive processes, tension, pressure, 

 \ \ etc., may modify this in such a way that almost any form 

 ) may be found: polygonal, flattened, elongated, fibrous, 



( branched, etc. 



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