36 Introduction 



The two principal regions of most cells are the nucleus and the 

 cytoplasm. The nucleus controls the reproduction of the cell while the 

 cytoplasm furnishes the material for cell metabolism. The individual 

 cell is surrounded by a semipermeable membrane that determines the 

 materials that enter and leave. The passage of the various ions and 

 molecules through this membrane is to a great extent determined by 

 the physical laws governing diffusion and osmosis. A great many more 

 important secondary structures are found in both the nucleus and cyto- 

 plasm. These will be discussed in a later chapter which is concerned 

 with cell division and other cell activities. 



BED BLOOD CELLS - FROG 



SMOOTH MUSCLE 



Fig. 3. — Examples of cells. 



Cell Sizes. — Cells vary greatly in size from the large bird egg, 

 with its inclusion of yolk, to the small blood cells. Most cells, how- 

 ever, are so small that they are measured in microns. A single micron 

 is 1/1,000 part of a millimeter. 



A few cells and their average sizes are listed below : 



Bacterial cell 



Human red blood cell 



Human ovum 



Human white blood cell 



Smooth muscle cell 



Skeletal muscle cells 



1-5 microns 

 7.2 microns 

 130-140 microns 

 12 microns 

 0.2 mm. 

 1-40 mm. 



Tissues. — In both animals and plants, cells which are similar 

 in structure and function are grouped together as tissues. Animal 

 tissues are divided into six principal classes. 



1. Epithelial Tissues. — Epithelial tissues (Fig, 4) cover sur- 

 faces such as those of the skin or digestive tract. This tissue has many 

 functions, chief among which are protection and secretion. In some 

 places, such as the lining of the nasal passages, the cells have cilia 

 or hairlike processes which remove dust particles. These cells are 

 known as ciliated epithelium. Epithelium is classified according to the 

 shapes of the cells composing it. Squamous epithelium consists of flat 



