CELLS AND THE CELL THEORY 



29 



1. The protoplasm, or cell substance, a liquid making up the 

 bulk of the cell. 



2. The nucleus, a rounded body within the cell substance. 



3. The centrosome, a small body near the nucleus. 



4. The cell wall, an outer covering which holds the cell 

 substance. (The cell wall and centrosome are sometimes 

 absent.) 



Size. There is much variation in the size of cells. Some of 

 them are extremely minute. 

 Bacteria, which are sometimes 

 not more than 1/50,000 of an 

 inch in diameter, are probably 

 cells (Fig. 7), although we do 

 not yet know positively that 

 they contain a nucleus and cen- 

 trosome. At all events the 

 yeast, which is only a little 

 larger than a bacterium (about 

 1/3000 of an inch), is a typical 

 cell, possessing a nucleus, cell 

 wall, and cell substance; see 

 Fig. 32. At the other end of 

 the scale we find giant cells, 



FIG. 7. BACTERIA VERY HIGHLY 

 MAGNIFIED 



Showing the complex internal structure with 

 bodies supposed by some to be nuclei. At a 

 one cell shows what resembles karyokinetic 

 division. 



FlG. 8. NlTELLA 



A, about natural size, showing nodes 

 and internodes; B, one of the inter- 

 nodes more magnified. The part en- 

 closed by brackets, between the two 

 rows of leaves, is a single cell. 



which may be an inch in length, as in the case of a small 

 plant known as Nitella (Fig. 8), or larger still, as in the egg 



