Cells and Their Activities 



267 



contrast microscope. This microscope increases the contrast of parts by- 

 special optical means in which differences of structures are converted into 

 differences visible to the eye. Phase microscopy thus is invaluable for 

 the study of living cells. 



THE STRUCTURE OF THE CELL 



The shape and size of cells varies greatly, depending upon the 

 function and the location of the individual cell. Nearly all, however, 

 have the same essential parts which may be variously modified. The 

 two most conspicuous regions of the cell are the nucleus bounded by the 

 nuclear membrane, and the cytoplasm with the plasma and cell mem- 

 branes at its outer limits. All the protoplasm of the cell contained 

 within the plasma membrane is known as the protoplast. Within both 

 these regions are many smaller structures which play a significant role 

 in the life of the cell. Only recently with special staining techniques and 

 microchemical analysis is the complexity of these minute structures 

 becoming more evident. 



CENTROSOMC 



JCENTROSPHERE 



[CENTRIOLE 

 NUCLEAR MEMBRANE 



.CELL MEMBRANE 

 GOLGI APPARATUS 



PLASMA MEMBRANE 



CHROMATIN 

 .CHROMONEMA 



MITOCHONDRIA 

 CYTOPLASM 



■CELLULAR INCLUSION 



Fig. 80. — Diagram of an animal cell. 



The Nucleus.— Usually the nucleus is located at the physiological 

 center of the cell, which is very often also the geographical center. In 

 the fixed, stained cell certain definite structures are clearly visible 

 within the nucleus. These include the nucleolus, the nuclear membrane, 

 and a series of interlaced filaments, the chromonemata. 



The chromonemata contain a granular substance, chromatin, which 

 stains brightly with certain coloring materials. Very often the indi- 



