INTRODUCTION 13 



visible. Vacuoles, plastids, and non-living bodies (metaplasm) 

 may also be present. The entire cell may or may not be sur- 

 rounded by a membrane. 



The cell nucleus contains a fluid through which runs a network 

 of thin linin fibers. Scattered about on these fibers are granules 



Attraction-sphere enclosing two centrosomes 



Plastids lying 

 in the cyto- 

 plasm 



Vacuole 



Passive bodies 

 (metaplasm or 

 paraplasm) 

 suspended in 

 the cyto- 

 plasm ic mesh- 

 work 



FIG. 2. Diagram of a cell. (From Wilson.) 



of chromatin, a substance that has a strong affinity for certain 

 dyes. Frequently several granules of chromatin unite to form 

 a net-knot or karyosome. In addition to these regular constit- 

 uents of the nucleus, one or more bodies, known as nucleoli, may 

 be present. In certain cases a cell may possess more than one 

 nucleus, and a few cells have no definite nucleus, but contain 

 chromatin granules which are scattered about in the cytoplasm. 

 (2) Physiology. There is a definite division of labor among 

 the parts of a cell. The particular function of the nucleus, aside 



X 



