STRUCTURE OF ORGANISMS. 23 



For the life of a cell both body and nucleus are essen- 

 tial. An interchange of material takes place between 

 them ; one cannot live without the other. If a cell is 

 divided into halves, only that which contains the nucleus 

 will continue to live, grow, and reproduce. The cell is 

 the smallest known unit of life. 



In the Bacteria there is no well-defined nucleus, the 

 chromatin being scattered. They probably represent a 



FIG. 1. Enlarged view of stained microscopic preparations to show the 

 structure and multiplication of cells and their nuclei in plant tissue 

 A (section through the tip of a root), and in animal tissue B (epi- 

 thelium of the gill of a larval Salamander), a, b, c, d represent 

 successive stages in the divisions of cells. 



primitive condition before the typical cell-structure was 

 completely differentiated. 



Among the lowest plants, Protophyta, and the lowest 

 animals, Protozoa, the whole individual consists of a 

 single isolated cell, which consequently has to perform all 

 the functions of life. Such organisms may, nevertheless, 

 become highly differentiated, developing special cell- 

 organs a mouth for taking in food, motile processes for 

 locomotion, contractile vacuoles for excretion, and so 



