PHYSIOLOGICAL 369 



When the thin sections are made, the paraffin or other material is 

 dissolved away, and some dye is used which will stain various parts 

 of the cell differently. In many cases two or three stains may be 

 used in succession to differentiate part from part more precisely. 

 Finally, the section is {a) thoroughly dehydrated in absolute alcohol, 

 {b) "cleared" of its fixative by soaking in benzol, clove-oil, or the 

 like, and (c) mounted in a suitable preserving medium, such as 

 Canada balsam. We have outlined this question of technique partly 

 that it may be quite clear what has been done to the cells which 

 have yielded the picture of detailed intricacy shown in book-figures, 

 and partly for another reason of even greater importance. There can 

 be no doubt that the methods of fixing and staining technique, of 

 which we have given only a glimpse, have greatly contributed to 

 the intimate modern knowledge of the microcosm of the cell. 

 Without differential staining we could have known little of the 

 complicated processes of cell-division and fertilisation, and a con- 

 siderable part of the modern theory of heredity depends on the 

 same possibihty (see "Chromosome Theory of Heredity"). At the 

 same time it has to be admitted that the technical methods by 

 their very perfection led to an erroneous conception of protoplasm. 

 A well-fixed and well-stained cell often shows an intricate visible 

 cytoplasmic structure, which has been described as reticular, 

 fibrillar, alveolar, and so forth. But it now appears that this visible 

 microscopic structure is the result of the artificial treatment the cell 

 has received. It can be induced, as Hardy showed, in perfectly 

 structureless fluid gelatine or albumin. Moreover, the same cell will 

 show different kinds of cytoplasmic structure according as it is 

 treated. Apart from granules and plastids there is no visible structure 

 in living protoplasm, though there is reason to believe in the 

 presence of ultra-microscopic intracellular films which divide the 

 cytoplasm into areas. These delicate films are occasionally to be 

 seen around vacuoles in a cell. 



Living protoplasm is a more or less viscous fluid in which innu- 

 merable ultra-microscopic granules and immiscible droplets are 

 suspended. In other words, living protoplasm is in a colloid state. 

 What are the evidences of this ? The protean changes of form in an 

 Amoeba point to the absence of any permanent solid structure in 

 the protoplasm, and so do the microdissection experiments w^hich 

 show that a needle can be drawn through living protoplasm without 

 having any effect whatsoever. The absence of anything like a net- 

 work is also proved by the simple observation that when there are 

 drops of water within a living cell, these always assume a spherical 

 shape. Another important fact is that minute granules in cells may 

 sometimes be seen to be in constant "Brownian movement", which 

 is due to their being continually buffeted by moving molecules of 

 the fluid, in accordance with the kinetic theory of liquids. This is 



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