CHAPTER III 



THE ANIMAL CELL; ITS MORPHOLOGY AND PHYSIOLOGY 



31. 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, — oi'gans, tissues, 

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

 which we have learned the mass of facts which we have at pres- 

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

 results of such studies, to start with the unit of structure which 

 is uniformly found, and, by a synthetic process, trace the build- 

 ing up of an organism from its elementary parts. This is the 

 process which the development of every individual illustrates. 

 It has the special advantage of emphasizing the fundamental 

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

 and gives, as no dissection can, the true significance of differ- 

 entiation and development. 



32. 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, as we shall see, a group of bodies all of which are in 

 turn highly complex. It is thus to be looked upon as an organ- 

 ized structure. 



33. Cell Form. — Cells, unhampered or not specially stimu- 

 lated as to the direction of their growth, tend to assume a spherical 

 form. Agencies, both internal and external, as nutritive proc- 



• esses, differences in tension, pressure, etc., may modify this in 

 such a way that almost any form may be found: polygonal, 

 flattened, elongated, filamentous, branched, etc. 



19 



