COLLEGE OF DENTISTRY 



UNIVERSITY OF CALIFORNIA 



CHAPTER XIII. 



THE ANIMAL CELL. 



THE cell constitutes the morphological unit of all animal and 

 vegetable life, and as such is capable of manifesting those peculiar 

 activities which we regard as characteristic of living matter. In 

 its simplest form it represents a tiny bit of a more or less granular, 

 gelatinous substance the so-called protoplasm in the interior of 

 which a somewhat more solid-looking body can be made out, which 

 is termed the nucleus. Such simple cells exist in nature, either as 

 such or as conglomerations of many cells which represent the higher 

 forms of animal and vegetable life. All living matter, however, 

 whether simple or complex, has for its origin the single cell. But 

 while in the lowest forms of life the single cell is capable of per- 

 forming all those functions which are characteristic of living matter 

 by itself, we find, as we ascend in the scale of animal and vegetable 

 life, that certain groups of cells are here set aside for the purpose 

 of executing separate functions. Such groups of cells we speak of 

 as tissues, and we accordingly find in the highly organized mammal 

 a differentiation of the entire body into tissues, which according 

 to their functions may be grouped as tissues of locomotion, of re- 

 production, of digestion, of excretion, etc. With such a differentia- 

 tion of cells into tissues, however, the original aspect of the cell is 

 more or less changed. The highly differentiated voluntary muscle- 

 cell would thus at first sight scarcely be recognized as being in any 

 way related to the oval cell from which it is primarily derived. 

 On careful examination, however, we find that, no matter how 

 unlike its ancestral cell such a specialized cell may appear, the dif- 

 ference is only apparent. The striated portion of the muscle-cell is 

 thus nothing more than the protoplasm of the original cell, differen- 

 tiated and modified in accordance with the function which the cell 

 is to perform. In some cells, however, such as those of the adipose 

 tissue, the original differentiation into protoplasm and nucleus is ap- 

 parently lost, and on ordinary microscopical examination it appears 

 that such cells are nothing but large globules of fat. But with 

 special methods of staining we can demonstrate even here that there 

 are a nucleus and protoplasm. The only cells, in fact, in which a 

 nucleus cannot always be demonstrated are the red corpuscles of the 

 circulating blood of the mammalia. We find, however, that even 

 in adult man all red corpuscles at one period of their existence, viz., 

 in their juvenile form, are nucleated, and that under certain condi- 



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