STRUCTURE AND ACTIVITIES OF CELLS 



tissue and grow as thin sheets in which their activities can be studied. In 

 organ culture very small organs are grown (Fig. 2.5). The reactions of their 

 cells can be studied as they exist normally with reference to their neighbors 

 in the organ but apart from the normal hormonal or nervous control of the 

 body (see Chap. 4). In both tissue and organ culture a group of normally 

 associated cells is removed for growth outside the body. Dissociation of cells 

 can be accomplished by gentle techniques that digest the material cementing 

 the cells together, much as mortar might be removed from a brick wall to 

 recover the individual bricks. Such dissociated cells can be studied as spread 

 on glass slides or as grown by methods of tissue and organ culture. 



A Composite Cell 



A cell may be correctly defined, in a general way, as a mass of protoplasm 

 containing a nucleus. Certain cells, however, notably the red blood cells of 

 mammals, lose their nuclei before they enter the blood vessels; others, such as 

 striated muscle cells, contain many nuclei. If the definition is to hold for 

 all cases, it must be modified to state that a cell is a mass of protoplasm which 

 contains one or more nuclei at some time in its life. This statement, together 

 with the definition of a cell as the unit of structure and function, is of funda- 

 mental importance in the further consideration of cells. Structurally, cells 

 are divided into two main parts, the cytosome, or cell body, and the nucleus. 

 In the following account these parts will be discussed as they occur in what 

 may be called a composite cell which is not dividing; that is, parts that can 

 be identified in many different kinds of cells are described in a single cell. 

 We shall explore this cell from the outside inward. 



The cytosome of an animal cell contains what is commonly referred to as 

 cytoplasm and is limited externally by the cell membrane, which is also called 



Watch-glass culture 



Organs Clot 



Wet 

 cotton 



Clot 



Fig. 2.5. An organ culture; diagrammatic. 



17 



