CHAPTER I. 

 THE CELL. 



As has been stated in the introductory chapter, the cells of the 

 body are not all alike. Most of them have undergone modifications 

 fitting them for the performance of some definite function, and the 

 majority of them are in consequence not appropriate objects for a 

 study of the general characters of a cell. The extent to which this 

 modification has affected the visible structure of the cell is, how- 

 ever, very different in the different tissues, and in some of them the 

 cells retain so much of their original embryonic appearance as to 

 closely resemble the unspecialized cell. 



This is true of the cells of some varieties of epithelium. But, 

 though in appearance they give little evidence of specialization, in 

 their functional activities they display very marked modifications of 

 the powers of the primitive cell. Some of those powers, perhaps 

 the nutritive, perhaps the secretory, have become exaggerated, while 

 others, e. g., the locomotor y, or reproductive, have fallen into abey- 

 ance, or suffered almost total extinction. 



On the other hand, it is obvious that such cells as constitute the 

 whole body of unicellular animals must retain all the powers essen- 

 tial to a living cell in relatively equal states of development. No 

 one of them can be extinguished or thrown out of its proper bal- 

 ance with respect to the others if the cell is to remain normal. 

 And yet, even among the unicellular organisms, certain parts of the 

 cell may be very evidently specialized for the performance of par- 

 ticular functions. For example, the cilia of infusoria have the 

 power of executing much more rapid movements than the other 

 parts of the same cell. And it is probable that all protozoa, i. e. 

 unicellular animals, possess similar, though less obvious and in- 

 ternal, heterogeneity of constitution. 



The less the degree of specialization or differentiation in the 

 structure of an organism, the less highly developed is the functional 

 activity of which it is capable, and the less perfect its ability to 

 cope with possible unfavorable environment. The value to the 



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