78 CYTOLOGICAL TECHNIQUE 



the appearance of the cells is astonishingly lifehke. Methacrylate 

 is seldom used when sections are to be dyed. It finds its chief 

 application in the preparation of very thin sections for electron- 

 microscopy. The main disadvantage of methacrylate is that the 

 process of embedding is rather complicated. 



Very small pieces of tissue (often about 1 cubic mm) are 

 generally used. A solution of osmium tetroxide, buffered at about 

 pH 7-4, is the most usual fixative (p. 58). After fixation the tissue 

 may be washed with distilled water (though many workers use 

 weak ethanol solutions), and must then be dehydrated. Graded 

 ethanols are used for this purpose. It has not been found neces- 

 sary, even for the most delicate work, to use a closely graded 

 series of ethanols. The series recommended for use in paraffin 

 embedding (50%, 80%, 96%, absolute) is suitable. No special 

 antemedium is required. The tissue may be transferred to a 

 mixture of equal volumes of absolute ethanol and methacrylate, 

 or directly to methacrylate itself. 



The embedding medium to which the tissue is transferred is 

 /7-butyl methacrylate, in monomeric form. This is a colourless, 

 mobile liquid, boiHng at about 163° C. It has a distinctive, rather 

 unpleasant smell. The vapour is somewhat toxic and it is desir- 

 able to increase the ventilation if the smell is noticeable. Butyl 

 methacrylate is miscible with absolute ethanol and with lipid- 

 solvents such as carbon tetrachloride, acetone, and toluene. It is 

 insoluble in water, on which it floats. It is highly inflammable. 



The acrylic acids form a series similar to the fatty acids, but 

 two of the carbon atoms in the chain are joined by a double 

 bond. In the strict sense the series includes only those acids in 

 which the double bond is in the same position, in relation to the 

 carboxyl group, as in acrylic acid itself: that is to say, just beyond 



H H 

 -C— C— 



Acrylic acid as a segmer 

 in a polymer 



