CHAPTER 3 



Cells and Tissues 



10. The Cell and Its Contents 



The living substance of all animals is organized into units called 

 cells. A cell is a mass ot protoplasm containing a nucleus and surrounded 

 by a plasma membrane. Mammalian red blood cells lose their nucleus in 

 the process of maturation, and a few types of cells such as those of 

 skeletal muscles have several nuclei per cell, but these are rare excep- 

 tions to the general rule of one nucleus per cell. In the simplest animals, 

 the protozoa, all of the protoplasm is found within a single plasma 

 membrane. These animals may be considered to be unicellular, i.e., 

 single-celled, or acellular, with bodies not divided into cells. Many 

 protozoa have a high degree of specialization of form and function 

 within this single cell (Fig. 3.1), and the single cell may be quite large, 

 larger tlian certain multicellular, more complex organisms. Thus, it 

 would be wrong to infer that a single-celled animal is necessarily smaller 

 or less complex than a many-celled animal. 



The term "cell" was applied by Robert Hooke, some 300 years ago, 

 to the small, box-like cavities he saw when he examined cork and other 

 plant material under the newly-invented compound microscope. The 

 important part of the cell, we now realize, is not the cellulose wall seen 

 by Hooke, but the cell contents. In 1839 the Bohemian physiologist, 

 Purkinje, introduced the term "protoplasm" for the living material of 

 the cell. At this time a German botanist, Schleiden, and Schwann, his 



Food. 



vacuolen 



Contractile 

 va.cuolen 



Oral disc 



Retractile 



Mou.th.->^ 



r 1 aminae Rectum 



•-Plasmasol] ^^^^ 



Motor 

 mass 



Contractile 

 vaCLtole 



Endoplasm. 

 MicronuclcLLS 



Macronucleus Ectoplasm 



Region of 

 gelat-ion 



"JEctoplasTTL 



Figure 3.1. Diagrams of an anieba (left) and Epidinium (right) to illustrate the 

 range in complexity of the single-celled animals. 



33 



