CHAPTER XXIV 



MORPHOLOGY OF THE CELL AND ITS MOVEMENTS 



ALL living beings are made up of cells. Some have but 

 one cell ; these are simplest in point of structure, at least, 

 there are very simple living beings among them. It is of 

 these we have to speak in this Second Book. 



The cell, as observed under the microscope, in spite of 

 great diversity in its specific form, can be reduced to a con- 

 stant type so far as its essential parts are concerned. Fig. 6 

 gives the diagram of this 

 general type. The living 

 protoplasm may be divided 

 into three essential parts : 

 the nucleus in the centre ; 

 around the nucleus the cyto- 

 plasm ; around the cytoplasm 

 the cell-membrane, which 



should often be considered a particular phase of the cyto- 

 plasm's periphery in contact with the surrounding medium. 



These are the living parts of the cell. Often, within the 

 cytoplasm, there may be observed enclosed foreign matter } 

 either the digestive vacuoles already mentioned or granules 

 of reserve substances. 



The cell-membrane is also often encrusted with not-living 

 substances ; sometimes even it is surrounded by a more or 



149 



+ Cell-membrane 



- Cytoplasm 



~~ Nucleus 



FIG. 6. 



