CELLS. 13 



It will be shown beyond that these ultimate units or cells 

 possess everywhere the same fundamental structure ; but they 

 differ immensely in form, size, and mode of action, not only in 

 different animals and plants, but even in different parts of the 

 same individual. As a rule, the cells of any given tissue are 

 closely similar one to another and are devoted to the same func- 

 tion, but differ from those of other tissues in form, size, arrange- 

 ment, and especially in function. Indeed, the differences be- 

 tween tissues are merely the outcome of the differences between 

 the cells composing them. The skin of the hand differs in ap- 

 pearance and uses from the muscle which it covers, because skin- 

 cells differ from muscle-cells in form, size, color, function, etc. 

 Hence a tissue may be denned as a group of similar cells hav- 

 ing a similar function.* As a rule, each organ consists of 

 several such groups of cells or tissues, but, as stated above, young 

 organs are nearly or quite homogeneous ; that is, all of the cells 

 are nearly or quite alike. It is only when the organ grows 

 older that the cells become different and arrange themselves in 

 different groups, a process known as the differentiation of the 

 tissues. In the case of some organs for instance the leaf of a 

 moss the cells remain permanently nearly alike, somewhat as 

 in the embryonic condition, and the whole organ consists of a 

 single tissue. 



What has been said thus far applies only to higher plants 

 and animals. But it is an interesting and suggestive fact that 

 there are also innumerable isolated cells, both vegetal and 

 animal, which are able to carry on an independent existence as 

 one-celled plants or animals. Physiologically these must cer- 

 tainly be regarded as individuals; but it is no less certain that 

 they are equivalent, morphologically, to the constituent cells of 

 ordinary many-celled organisms. It will appear hereafter that 

 the study of such unicellular organisms forms the logical ground- 

 work of all biological science. (See p. 157.) 



Since organisms may be resolved successively into organs, 

 tissues and cells, it is evident that cells must contain living 

 matter. And a cell may be denned as a small mass of living 

 matter either living apart or forming one of the ultimate units 



* Tissues frequently contain matters deposited between cells ; but these 

 have usually been directly derived from the cells, and vary as the cells vary. 



