22 CYTOLOGICAL TECHNIQUE 



Structure before autolysis has damaged it. In designing fixative 

 mixtures it is helpful to know the relative rates of penetration of 

 the components, for ideally one component should not outstrip 

 another. 



To study the rate of penetration of fixatives, it is best to begin 

 by using a homogeneous protein gel in place of a piece of 

 tissue. ^^"' ^^ A suitable gel may be made by dissolving gelatine 

 at 15% in warm white-of-egg.^^ This material may be used as a 

 crude model of cytoplasm, for its refractive index (and hence its 

 protein-content) is about the same. While still warm it can be 

 drawn into glass tubes, and the gel then allowed to set. If one end 

 of the tube is inserted into a solution of a coagulant fixative, the 

 rate of penetration can easily be noted, since the material becomes 

 white and opaque on coagulation. To find how far a non- 

 coagulant fixative has penetrated, it is only necessary to turn the 

 tube upside down in a bowl of warm water, for the unfixed gel 

 then runs out, while the fixed material remains in the tube.^^ It 

 will be understood that what is measured in these experiments is 

 the distance through which the substance under test has moved at 

 the concentration that is just sufficient to fix the gel. 



Fixatives penetrate into the gel rapidly at first and progres- 

 sively more slowly as time goes on. It has been shown that nearly 

 all fixatives move forward in accordance with the laws of 

 diffusion. ^^° If d is the distance penetrated in time /, and ^ is a 

 constant depending on the fixative used, then 



d =K\/t. 



Distance may be measured in mm and time in hours. K then 

 represents the distance in mm through which the fixative has 

 moved forward in one hour, at a concentration sufficient to fix 

 the protein. 



It is convenient to transform the equation just given into its 

 logarithmic form, 



log d ^\ogK + ^log /. 



This, when graphed, will necessarily give a straight line. If a 

 set of figures for <i and /, obtained in an experiment, are converted 

 into logarithms and marked on a graph, it will at once be seen 



