136 FIXATION 



Reactions zvith carbohydrates. Acetic acid neither fixes nor 

 destroys carbohydrates. 



Rate of penetration. Acetic acid penetrates at moderate speed into 

 gelatine/nucleoprotein gel (K = 2-75). It may be remembered that 

 its rate of penetration into gelatine/albumin gel cannot be measured, 

 because it does not fix this gel. For the same reason the rate of its 

 penetration into tissues cannot be measured in such a way as to 

 give a K- value comparable with the others quoted in this book (see 

 under potassium dichromate, p. 131). No doubt it runs quickly 

 through the tissues, as Tellyesniczky's data suggest (K = 1-2), but 

 it penetrates without fixing proteins, precipitating nucleic acids as 

 it goes. 



Shrinkage or swelling. Acetic acid swells protein gels far more 

 than any other fixative, for reasons that have been discussed (p. 

 64). A simple aqueous gelatine gel (15% w/W), placed in acetic 

 acid solution, expands to about 13 times its original volume in a 

 week. Under the standard conditions of measurement, gelatine/ 

 albumin gel expands to 455% of its original volume (see fig. i, p. 

 36). Tissues and cells also swell in acetic acid, but if they are not 

 stabilized in the swollen state by the action of some other fixative, 

 they shrink strongly on dehydration and subsequent treatment. 

 Thus the spermatocytes of the snail retain only 28% of their 

 original volume when paraffin sections have been mounted in 

 Canada balsam. They retain a considerably larger volume if 

 formaldehyde be used as fixative, though this causes very much less 

 initial swelling of protein gels. 



Hardening. Acetic acid leaves tissues much softer than any other 

 fixative. Wetzel's figure for rigidity is only about 9. The figure for 

 chromium trioxide is 25 times as great. After subsequent soaking 

 in 80% alcohol, tissues remain extremely soft. 



Immediate effects on particidar constituents of the cell. The cell- 

 outline becomes rather indistinct; any thin pseudopodia tend to be 

 transformed into rows of globules; ground cytoplasm loses its 

 original homogeneity; mitochondria are transformed into faint 

 rows of granules ^^^' *^^ and generally disappear; lipid globules 

 are sometimes well retained, but the neutrophil granules of 

 polymorphs disappear. The nucleus sometimes retracts from 

 the cytoplasm; the nuclear contents are transformed into a 

 lumpy network; the nucleolus sometimes becomes irregular in 

 shape. 



Methods of washing out. Since acetic acid is perfectly miscible 



