32 THE CELL, ITS STRUCTURE AND FUNCTIONS 



microscopically. They vary in size and may measure 

 from ^ViT of an inch, the diameter of a red blood cell, 

 to Y-J-tf of an inch the diameter of the large cells in 

 the gray matter of the spinal cord. The structure of 

 a cell consists of a gelatinous substance, usually homo- 

 geneous, called protoplasm or cytoplasm, containing 

 a small spheric body, the nucleus, which latter con- 

 tains the nucleolus. Young cells appear clean, mature 

 cells contain, depending on the tissue they are found 

 in, different substances, e. g., fat-globules, granules of 

 glycogen, mucigen, pigments, and digestive ferments. 

 Cells possess the power of changing their shape, and 

 are also capable of growth, nutrition, and reproduction. 



Growth. Newly reproduced cells are very small, 

 but they soon grow, owing to their characteristic 

 organization and surrounding medium, to resemble 

 the normal adult cell of a given tissue. 



Nutrition. Cells not only must grow, but they have 

 to repair or make up the loss from waste, etc. Growth 

 and nutrition are dependent not only upon the power 

 possessed by living material of absorbing its nutrition 

 from the lymph, but also upon the property of taking 

 that nutrition and converting it into material similar 

 to its own, before waste took place, and then endow- 

 ing it with physiologic functions. Thus we have a 

 cell doing work, wasting as a result of such labor; 

 repairing not only its own body, but renewing its 

 powers of doing fresh work. 



Reproduction. Cells reproduce themselves by two 

 methods, direct and indirect division. (See Figs. 2 to 

 16, pages 36 to 39.) 



Direct division is seen when the nucleus of cells 

 becomes narrowed and divides with a grouping of the 

 nuclear elements. This is believed to occur only 

 where cell disintegration occurs. Indirect division 

 this is called karyokinesis is a complex process and 

 its main feature is due to the centrosome of a cell 

 becoming enlarged and in leaving the nucleus lying 



