24 CYTOLOGICAL TECHNIQUE 



penetrating into gelatine-albumin gel, the K-va\ue is 2-27. Form- 

 aldehyde shows the highest ^- value (3-6) among the seven selected 

 primary fixatives. Chromium trioxide is slow {K = 1-12). 



It is convenient to make the units of the abscissa-scale (log t) 

 one-half of those of the ordinate-scale (log d). Then, if the frac- 

 tion before log t in the logarithmic equation is ^ (that is, if d is 

 proportional to the square root of t), the slope of the line will be 

 45°. This is so with most fixatives (see fig. 3). 



Few tissues are sufficiently homogeneous to give clear-cut 

 results in tests of rate of penetration. Liver, however, is suitable, 

 and certain coagulant fixatives leave a clear indication of how far 

 they have moved into this organ, since the fixed tissue is whitish 

 and opaque. Experiments with mercuric chloride show that here 

 also the laws of diffusion are obeyed. The X^-value of mercuric 

 chloride diff'using into liver is 0-84. So far as is known, all fixatives 

 penetrate much more slowly mto liver than into gelatine-albumin 

 gel, perhaps because they are held back by the lipids of cell- 

 membranes and ground cytoplasm. There is, however, a general 

 agreement in the results obtained with gels and liver. Thus 

 chromium trioxide penetrates much more slowly than mercuric 

 chloride into both. 



It is important to realize how rapidly the rate of penetration 

 of fixatives falls off" with time. With a ^-value of 0-84, mercuric 

 chloride penetrates into liver a distance of 20fx (the diameter of a 

 large cell) in just over two seconds. That is to say, it penetrates 

 this small distance at the rate of 36 mm an hour; but actually it 

 only penetrates 0-84 mm in one hour, and it would take 77 days to 

 penetrate 36 mm. These facts emphasize the importance of using 

 small pieces of tissue for fixation. Pieces several cm thick are only 

 adequately fixed in their external parts, unless they are perfused 

 with the fixative. 



Instructions are often given as to the time during which a 

 particular fixative should act, but it is usually safe to disregard 

 these. For light-microscopy it is convenient as a general rule to 

 fix overnight (say 18 hours). Little difference will in many cases 

 be noticed if the period is extended to several days, or even weeks 

 or years. A shorter time than 18 hours suffices for the pieces used 



