72 CYTOLOGICAL TECHNIQUE 



disadvantages of this medium are the shrinkage that occurs 

 during embedding, and the solution of most lipids. Indeed, the 

 incompleteness of our knowledge of the lipid constituents of 

 cells must be attributed largely to the popularity of paraffin 

 among microtomists. 



To prepare tissues for infiltration by melted paraffin wax, it is 

 necessary to dehydrate them thoroughly and soak them in a fluid 

 that is readily miscible with the embedding medium. Ethanol and 

 cellosolve are particularly suitable dehydrating agents. 



If all the constituents of the fixative are freely soluble in or 

 miscible with 50% ethanol, the tissue may be transferred directly 

 to that fluid. If one or more of the constituents of the fixative 

 react with ethanol to produce insoluble material, it is usual to 

 wash the tissue thoroughly in running water after fixation. 

 Chromium trioxide, for instance, tends to be reduced by ethanol 

 to insoluble green chromic oxide, CroOg; osmium tetroxide to 

 black or brown dioxide (OsOa or Os02.2HoO). It is worth re- 

 marking, however, that tissues fixed in chromium trioxide solu- 

 tion may be transferred directly to 50% ethanol, if 2% v/v of 

 strong sulphuric acid has previously been added to the alcoholic 

 solution.^* 



It is customary to transfer tissues through a series of aqueous 

 alcoholic solutions of higher and higher ethanol content, until 

 absolute ethanol is reached. A suitable series is 50%, 80%, 96%, 

 absolute. Many more grades than these are often used, and 

 special apparatus has been invented to achieve very gradual 

 dehydration. It is doubtful whether any benefit accrues from this. 

 However closely graded the series of ethanol solutions may be, 

 there is always considerable shrinkage at one concentration or 

 another, somewhere between 60% and 90 °o ethanol. ^^ It is a 

 useful experience to judge the appearances of dyed and mounted 

 sections of tissues that have been dehydrated in diff"erent ways. A 

 friend may be asked to fix two pieces of the same organ in the 

 same fixative, and to dehydrate them in two diff"erent ways : one 

 piece through a closely graded series of ethanols, the other 

 through a coarsely graded series, or even from the washing water 

 directly to absolute ethanol. If he makes a number of slides from 



