PROTOPLASM AND THE CELL 



25 



or collections, the cells of which are alike in size and shape Such 

 a collection of cells is called a tissue. Examples of tissues are the 

 cells covering the outside of the human body, the cells which col- 

 lectively allow of movement, the so-called muscles; the material 

 that forms the framework to which the muscles are attached, the 

 bony tissues; and many others.^ 



Cells. — A cell may he defined as the smallest bit of living matter 

 that can live alone. All plant and animal cells appear to be alike 

 in the fact that every living cell possesses a structure known as 

 the nucleus, which is found within the body of the cell. The 

 nucleus is composed of q 



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living matter like the rest 

 of the cell, although it 

 seems to differ in some 

 chemical way from that 

 part of the cell surround- 

 ing it. This is seen when 

 a plant or animal is placed 

 in a liquid containing 

 some dye such as log- 

 wood. Certain bodies in 

 the nucleus take up the 

 stain much more readily 

 than the rest of the liv- 

 ing matter of the cell, 

 taking on a deep black 

 color. They are thus 

 called the chromosomes 

 (color-bearing bodies).^ 



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Diagram of a cell (after Wilson). The cell protoplasm 

 contains cell food (C./.) ; spaces contain liquid cell 

 sap {C.s.) ; just above the nucleus (iV./.) is a struc- 

 ture called the centrosome (c\ which aids in cell 

 division; within the nucleus are chromosomes 

 (N.n.), which form a network; t.n., nucleolus. 



1 A very simple tissue that may be studied as an introductory study with the 

 microscope is a thin section of elder or pith, mounted in water or glycernie (dilute) 

 on a glass slide. (See Hunter and Valentine, Manual, page 7.) The study of yeast 

 may be introduced at this point if microscopes are available; at any rate demon- 

 stration materifil showing isolated cells and tissues should here be shown. 



2 The chromosomes, which are believed to be always definite in number for every 

 tissue cell, are of much interest to scientists. It is found that each time a cell 

 splits to form two new cells, the chromosomes split in half and an equal number 

 of parts go into the nucleus of each new cell thus formed. _ These chromosomes are 

 supposed to be the bearers of the qualities which we believe can be handed down 

 from plant to plant and from animal to animal ; in other words, the mheritable 

 qualities which make the offspring like its parents. 



