CELLS AND THEIR ASSOCIATION 43 



which oxygen is consumed and simple end-products 

 formed. Movement in the independent cell, as in the 

 larger organisms, is the expression of contraction. Some 

 explanation of this term is desirable. 



Contraction, as the biologist understands it, does not 

 mean diminution of volume. It is thus different from 

 contraction in the physical sense. When the mercury 

 in the thermometer tube contracts under the influence 

 of cold there is an actual reduction of the space which the 

 metal occupies. When a muscle contracts it can be 



FIG. 3. The purpose of this diagram is to assist the student in 

 gaining notions of scale. The large circle stands for the cross-section 

 of a fine hair. Its diameter is supposed to be 3^00 inch. Within it 

 (bl) is a red blood corpuscle, ^200 i n( 'h across. The budding yeast-cell 

 (y) is of a similar order of magnitude. Bacteria (bact.) are a good deal 

 smaller. 



shown that the volume is unchanged; there has been a 

 shortening in one dimension but a compensating thick- 

 ening in others. Changes of form which are observed in 

 single cells are doubtless of this kind, save in those 

 cases in which water enters or leaves. This exceptional 

 possibility we shall not be obliged to consider in the 

 present treatment of the subject. 



Many of the facts of life which we assume for free- 

 living cells can hardly be demonstrated but are rather 

 inferred from what we know of higher organisms. The 



