28 



BIOLOGY 



ture of living things has been carried at present; for while each 

 cell is made up of parts, life as a whole seems to be found only 

 where we have the whole structure of the cell developed. In 

 other words, the cell is the simplest form in 

 which life occurs, and is, in this sense, the ulti- 

 mate unit of living structure. While an organ 

 may contain many different kinds of cells, each 

 tissue is, as a rule, made of but one kind of cell. 

 The cells of the bone, for example, are all essen- 

 tially alike, and so, too, are the cells of muscles 

 and glands. The different cells in the same tissue 

 may differ in shape and size; but these differences 

 are only superficial ; fundamentally the cells form- 

 ing a single tissue are alike. Therefore, if we de- 

 fine a cell as the ultimate unit in the analysis 

 of living structure, we may define a tissue as an 

 aggregate of similar cells, all having similar func- 

 tions; see Figs. 4, 5, and 6. 



While the form, structure, and size of cells 

 present an almost endless variety, in both the 

 animal and plant worlds, nevertheless, all cells 

 have in common certain general parts. Thus we 

 may speak of the structure of a cell in general, recognizing that 

 all living cells of both animals and plants, in spite of their 

 differences, conform essentially to the type of an ideal cell. 



FIG. 6. CELLS 



FORM ING 

 MUSCLE TIS- 

 SUE FROM 

 THE INTES- 

 TINE WALL 



CELL STRUCTURE 



The description given below is not that of any particular cell, 

 but rather that of a typical or ideal cell. Though a cell exactly 

 like that described will not be found, it resembles closely the 

 cell which forms the egg of certain animals, and in essential 

 structure is like all cells found in animals and plants. 



Structure. The cell consists of four primary parts, some of 

 which may be absent: 



