28 GENESIS OF MAN. 



this is itself very slight. Chemically, they can scarcely be dis- 

 tinguished. Both consist of a carbon compound, containing a 

 certain proportion of nitrogen, and belong to the albuminous group, 

 of which all animal tissues are principally composed. The nucleus 

 is generally of a darker color, but sometimes of a lighter, and may 

 or may not contain in its centre a minute dot — nucleolus. It also 

 may or may not be surrounded by a membranous envelope. This 

 is generally present in the cells of plants, while it is generally ab- 

 sent in those animals. 



The form of cells differs according to the circumstances of their 

 existence. They are the most plastic and easily modified by ex- 

 ternal conditions of all organized beings, and therefore make the 

 best subjects for the study of the law of adaptation. Stationary 

 cells in a motionless medium are uniformly spherical. When sub- 

 jected to pressure they assume hexagonal, elongated, or compressed 

 forms, according to the nature of the pressure. Cells that are 

 active in a liquid medium have a portion of the matter composing 

 their outer parts extended into a caudal appendage away from the 

 direction of motion. In addition to the forms named, cells fre- 

 quently assume others, sometimes taking wholly amorphous shapes, 

 resulting from the particular conditions to which they may be sub- 

 jected. They frequently change their form, and this not only from 

 external influences, but in obedience to internal or subjective de- 

 terminations. For a cell is a living creature. It possesses all the 

 essential characteristics of an organized individual. The only 

 functions necessary to characterize a living being are nutrition and 

 propagation. Both these the cell possesses. It grows by the 

 absorption of nourishment from the medium in which it lives. 

 Where this nourishment is not uniformly mingled throughout the 

 medium, but exists in the form of scattered solid particles, the cell 

 acquires the power to extend portions of its substance into tempo- 

 rary organs of grasping {pseudo podia), and thus to enclose and 

 devour its food. It thus improvises a mouth and jaws on whichever 

 side it may need them, and feeds itself after the manner of another 

 animal. 



The cell propagates, like many much higher organisms, by 

 division, or fission. It continues to take nourishment and to grow 

 until it reaches the limit fixed by heredity for its size, and then, 

 instead of growing larger or of ceasing to take food, it divides into- 



