ZOOLOGY 



Cell life and 



individual 



life 



2. In protoplasm the molecule is composed of in- 

 numerable atoms, the colloid particle of many mole- 

 cules ; tfie cell, of multitudes of these colloid particles. 

 System within system, they all function as a unit ; 

 and the individual animal or plant, made up of mil- 

 lions of cells, also behaves as a single machine. Never- 

 theless, the cell is a definite unit of life, and its indi- 

 viduality is not lost in that of the creature of which it 

 forms a part. During the life of the individual, cells 

 are born and die ; every time we wash our hands, dead 

 skin cells, so small and flat as to escape observation, 

 fall away. In the blood are active cells known as 

 leucocytes (Greek for "white cells") or white blood cor- 

 puscles, which crawl about with a flowing motion, 

 looking like certain free single-celled animals (Protozoa) 

 which are found in the water of ditches (Fig. i). These 

 leucocytes may be taken from the body, and if kept in 

 a nourishing solution at the right temperature, continue 

 to live as independent beings. Still more remarkable 

 is the fact, recently discovered, that portions of a liv- 



m body, composed of 

 highly specialized cells, 

 may be cut off and 

 isolated, and under 

 suitable conditions will 

 go on growing for an 

 indefinite period. No 

 one can deny life to 

 such isolated particles ; 

 yet the admission com- 

 pels us to recognize 



FIG. i. Amiba, one of the Protozoa ; an ex- that the "life" of a 

 ample of a free-living cell, occurring in ponds - j 



and ditches, n, nucleus; ,, ec, inner and man 1S Composite, and 



outer protoplasm ; p, pseudopodia. is in a true Sense the 



