130 



GENERAL ZOOLOGY 



Centrosome 



Chromatin 



Vacuole 



Nucleus 



Cytoplasm 



rounded in shape, which is called the nucleus (Fig. 70). 

 The more fluid protoplasm around it is called cytoplasm. 

 The single unit of protoplasm, the cell, has the power of 

 carrying on all the activities of the living animal, but when 

 many cells are united the various functions of living animals 

 are distributed among them. 



Tissues. In all animals, except the very simplest micro- 

 scopic forms, the cells in different parts of the body are 



specialized for carry- 

 ing on certain kinds 

 of work for the whole 

 body. Thus there is 

 a division of labor; 

 some cells, for exam- 

 ple, do the moving for 

 all the body and are 

 termed muscle cells. 

 When these cells of 

 similar function are 

 grouped together they 

 are made up of what 

 is known as a tissue. 

 The muscle tissue, 

 with which all are familiar in the form of a piece of lean 

 beefsteak, is composed of countless numbers of muscle cells. 

 The organs which have been discussed in the preceding chap- 

 ters as the conspicuous structures within the body are made 

 up of various tissues. In these tissues the cytoplasm of the 

 individual cells becomes modified in form and structure for 

 carrying on limited functions. 



Function of the Nucleus. The nucleus seems to control the 

 activity of the cell. Within the nucleus there is a substance 

 that is very important because it has the power of passing 

 from one generation to the next those features and condi- 



•% 





Stored 

 material 



Fig. 70. Diagram of an animal cell. 

 (Greatly magnified) 



