72 



ELEMENTS OF BIOLOGY. 



in infusions containing organic matter, are, as we shall sub- 

 sequently see, inconceivably minute molecules. In this, as 

 in other cases, the molecules are to be regarded in all pro- 

 bability as being small masses or centres of bioplasm, which 

 may or may not be surrounded by a proper wall. 



CELLS. 



If we regard a little mass or spherule of bioplasmic matter 

 as being the primitive and simplest life-element, it never- 

 theless is very rare to find this primordial condition un- 

 complicated or retained throughout life. In the Forami7ii- 

 fcra and Monera we may, perhaps, consider that we have 

 the nearest approach to this elementary state of things, 

 since the body in these degraded organisms consists simply 

 of a mass of structureless bioplasm, in which there is no 

 differentiation into definite parts. In the majority of cases, 

 however, changes take place in the primitive mass of bio- 

 plasm, by which it is converted into what is known as a 

 cell. In some plants, hence termed " unicellular," a single 

 cell constitutes the entire organism. In such cases, as in 



the Yeast-plant (fig. 19), a 

 complete individual may 

 be regarded as composed 

 of one cell, since in this 

 resides the power of both 

 nutrition and reproduc- 

 tion. In the majority of 

 cases, however, the organ- 

 ism is composed of a con- 



F5g. 19. —Cells of the Yeast -plant (/"^j^^/rt gCricS of CClls, Cach of 

 cerevisicE), greatly magnified. The shaded •, • •■ 



portions represent the bioplasm, coloured wllich CnjOyS tO a Certain 

 by carmine. ^ ^ , . - - , 



extent a fife of its own, 

 whilst its existence is, nevertheless, bound up with that of 

 the whole. Not only is this the case, but in many instances 

 the cells which form the organism are so modified that they 

 constitute special tissues, such as muscular tissue, cartilage, 



