CHAPTER VII 

 CELLS 



Living protoplasm always exists in the form of minute masses known 

 as cells, which possess a characteristic structure. Organisms may consist 

 of but one cell or of many, but in either case the cell may be considered 

 the unit of structure, or the morphological unit. 



38. Definition. — A cell may be defined as a mass of protoplasm in 

 which can be distinguished a portion called the nucleus. A distinction 

 may be drawn between the substance of the nucleus, which is termed 

 nucleoplasm, and the protoplasm of the rest of the cell, which is called 

 cytoplasm. 



39. Sizes and Shapes of Cells. — Cells vary greatly in size. The most 

 minute animal cells are one-celled blood parasites which are invisible, 

 or only barely visible, under the highest powers of the microscope. 

 Most cells cannot be seen by the unaided eye. There are cells, however, 

 which are relatively gigantic. A one-celled organism, parasitic in the 

 alimentary canal of the lobster, reaches a length of two-thirds of an inch ; 

 and egg cells, with the yolk which they contain, may even exceed this in 

 diameter and contain a much greater amount of substance. Some nerve 

 cells, the main cell body of which is not proportionately very large, 

 possess fibers, which are parts of the cells, that may even reach a length of 

 several feet. 



Cells also vary greatly in shape. The typical form, unaffected by 

 environment or unmodified for the production of any particular function, 

 is spherical, but the pressure of adjacent cells or from other structures 

 may crowd these cells into a variety of shapes, such as polygonal, cubical, 

 columnar, or flat and platelike. Other cells, particularly muscle cells, 

 become greatly elongated and assume the form of fibers, while still others 

 become very complexly branched (Fig. 8). 



40. Numbers of Cells. — As has been previously stated, an organism 

 may consist of but one cell; however, most organisms are made up of 

 more, the numbers in the largest organisms running into the trillions. 



41. Structure of Cells. — A cell (Fig. 7) consists of a mass of jelly- 

 like cytoplasm inclosing a nucleus. The surface of this cytoplasm is 

 covered by a plasma membrane, or cell membrane, which is living and 

 semipermeable. Outside it may be a cell wall composed of material which 

 is not protoplasmic and is nonliving, being a secretion formed by the cell. 

 In animal cells this wall is often absent. 



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