THE CELL AND THE CELL THEORY 29 



As has already been noted, the size arid form of cells are variable. 

 Bacteria are among the smallest known, some of them measuring 

 not more than one hundred thousandth of a meter in length. 

 On the other hand, the eggs of a large bird are single cells; they 

 are exceeded in length by certain nerve cells whose fibers extend 

 .out almost a meter. The limit in size is probably determined 

 by the nucleus, since it regulates the metabolic activities of the 

 cell. 



Some idea of the wide variation in the form of cells may also 

 be gained from Figure 48, which shows specimens of various types. 



All of the life processes of the simplest animals, the Protozoa, 

 are carried on by a single cell. The cells of a multicellular ani- 

 mal, a Metazoon, are mutually dependent, and cannot exist if 

 isolated. In many cases there is actual continuity between the 

 protoplasm of different cells, and some zoologists have gone so 

 far as to maintain that the body of a complex animal should not 

 be considered an assemblage of separate cells, but a continuous 

 mass of protoplasm with nuclei scattered through it. Often 

 this connection between cells is brought about by slender proto- 

 plasmic bridges (Fig. 46, C) ; in other instances cells are so indis- 

 tinguishably blended that no boundaries can be discerned. A 

 cell is therefore not always a true " unit," but a necessary part 

 in a complex whole. The number of cells associated to form 

 a single organ is often enormous; the number of nerve cells in 

 the gray matter of the human brain has been estimated at 

 9,200,000,000; all of these together would occupy only a trifle 

 more than a cubic inch of space. 



2. CELL DIVISION 



Cells multiply either by direct or indirect division. Indirect 

 cell division always involves a definite series of stages which 

 follow each other in a regular sequence, the whole process being 

 known as mitosis, or mitotic cell division. Direct division or 

 amitosis, is a more simple process having no complex series of 

 stages. 



