EMBEDDING 77 



paraffin sections are very seldom dyed in the loose state. Egg- 

 white is an excellent adhesive for this purpose. It is best to make a 

 stock solution by diluting it with an equal volume of l°o aqueous 

 sodium chloride solution; any insoluble material is thrown 

 down by centrifuging. In the presence of sodium /7-hydroxy- 

 benzoate at 0-2% w/v, this solution resists the attack of bacteria 

 and moulds and may be kept for an indefinite period." For 

 use, the stock solution is diluted with about 50 times its volume 

 of distilled water. The fluid is then spread on a glass slide and 

 the section floated on it. The slide is warmed on a hot plate 

 sufficiently to soften but not melt the paraffin, and any folds in 

 the section flatten out. The water is then drained off as far as 

 possible and the slide left on the hot plate to dry. When it has 

 dried, the minute quantity of albumen between the section and 

 the glass suffices to stick them firmly together. 



Glycerine is often mixed with egg-white in the preparation of 

 the adhesive. ^^^ It serves no purpose." 



To remove the paraffin from the dried slide, it is only necessary 

 to soak it for a few minutes in xylene. The slide is then passed 

 through absolute ethanol to 90 °o and then 70 °o. If the dye that 

 is to be used is dissolved in a weak ethanol solution, the slide 

 may be transferred to it from 70 ''o ethanol; if the dye is dissolved 

 in water, it is best to rinse in distilled water first. 



BUTYL METHACRYLATE 



Butyl methacrylate was introduced into microtechnique as an 

 embedding medium in 1949. i^' AUhough modern plastics of 

 other kinds have been tried since then and one or two of them are 

 promising, yet the methacrylates are still among the best, at any 

 rate for routine use. 



The great advantages of this embedding medium are that it 

 causes little distortion and permits sections to be cut at any 

 thickness from 8 /< or more down to about 10 m/i (though for 

 very thin sections a mixture of butyl and methyl methacrylates is 

 used). If tissues are suitably fixed, sectioned in butyl methacrylate 

 at about 3 //, and mounted in an appropriate medium (p. 128), 



