28 AN INTRODUCTION TO ZOOLOGY 



arranged in concentric zones, each containing a different kind of 

 protoplasm; or it may be modified to form hair-like projections 

 called cilia, and other cellular organs. The structure of cyto- 

 plasm is, however, rather uniform in the same cell, but highly 

 variable in different cells, and sometimes even in different stages 

 of an individual cell. A careful examination of certain cells 

 appears to leave no doubt but that a reticular network is present 

 in the cytoplasm. Other cells show no sign of such an arrange- 

 ment of materials. The alveolar condition exhibited by many 

 cells is perhaps the most typical. 



Although a definite cell wall is more often found in plants than 

 in animals, the cytoplasm of the latter may secrete a delicate 

 membrane, or even a wall of considerable thickness, as in the case 

 of cartilage (Fig. 48, C). 



Embedded in the cytoplasm, usually near the nucleus, is a 

 minute but important body, the centrosome, or a pair close to- 

 gether. The centrosomes are often surrounded by a clear space, 

 the centrosphere. Among the other bodies suspended in the cyto- 

 plasm may be mentioned oil and water vacuoles, crystals, and cer- 

 tain organs known as plastids. Plastids may be colored (chromo- 

 .plastids) or white (leucoplastids). 



There is a definite division of labor among the parts of a cell. 

 The particular function of the nucleus, aside from its important 

 relation to cell division, to be described later, seems to be the 

 control of the activities by which the protoplasm is elaborated. 



The cytoplasm, from its direct relation to the outside world, is 

 the seat of such functions as irritability, absorption, digestion, 

 excretion, and respiration. The centra some is of importance 

 during cell division. The cell covering may be secreted for pro- 

 tection or support, or may be extremely delicate and have sig- 

 nificance only as it helps to control the absorption of certain 

 fluids. Plastids may represent stored food or waste products; 

 some of them, however, have other functions, e.g., the chloroplasts, 

 which carry on photosynthesis in many plant and a few animal 

 cells. 



