CELLS 



39 



the structures commonly present. The cytoplasm is usu- 

 ally made up of fibers, granules, and fluid plasma. Other 

 bodies such as crystals, oil globules, and water vacuoles, 

 may be present. Such " extra" bodies are called plastids, 

 metaplasm, etc. The nucleus is covered by a membrane 

 and has, besides some fluid plasma, a network of linin 

 fibers to which are attached irregular masses of chromatin. 

 Just outside the nucleus is the centrosphere which contains 

 two centrosomes. 



CELL 



MEMBRANE: 



NUCLEAR _ i 

 MEMBRANE. 



PLASTIDS'- 



CYTOPLASMIC 

 ' NETWORK 



FIG. 24. A typical cell showing all the parts. 



There is a division of labor among the structures men- 

 tioned. The plasma membrane on the outside of a cell 

 is semi-permeable, allowing certain substances to pass into 

 or out of the cytoplasm but not admitting others. The 

 cytoplasm does the work which is the particular task of 

 each cell. In a muscle cell it is specialized for contraction; 

 in a nerve cell it carries the nervous impulses. The cyto- 

 plasm may also store food or even waste products. The 

 general function of the nucleus appears to be the control 

 of synthetic metabolism. A cell without a nucleus may 

 work until its reserve energy is exhausted, but it can never 

 build up any new material. The chromatin in the nucleus 



