32 MATERIALS AND PROCESSES OF SLIDE-MAKING 



impossible to get a good section of a 72-hour chick embryo, for example, 

 using this shortened dioxane technique. 



Clearing agents, which remove the dehydrating agent from the tissues and 

 leave them in condition either for mounting in balsam or embedding in par- 

 affin, are of two main types. For mounting in balsam, it is customary to use 

 one of the essential oils or their synthetic equivalents. The advantage of these 

 materials is that many of them are readily miscible with 90 per cent to 95 per 

 cent alcohol and, therefore, are capable of removing the last traces of water 

 which may be left in the specimen after imperfect dehydration. 



In the author's opinion, the best clearing agent for general use before mak- 

 ing wholemounts is terpineol ("synthetic oil of lilac"). This material is readily 

 miscible with 90 per cent alcohol and has the additional advantages that it 

 has neither an unpleasant odor nor does it render objects brittle. It is, how- 

 ever, more customary today to recommend clove oil. This has the advantage 

 of being much more fluid than terpineol and the disadvantages of a very pungent 

 odor and the tendency to make small objects brittle. The latter is sometimes 

 an advantage, as when one is endeavoring to remove appendages from small 

 arthropods, but on other occasions it is very annoying. Many other oils have 

 been recommended from time to time, but these two between them will be 

 satisfactory for making a wide variety of wholemounts. Under no circum- 

 stances whatever should an essential oil be used to clear objects intended for 

 embedding in paraffin, because the oil is almost impossible to remove com- 

 pletely and will destroy the good cutting qualities of the embedding medium 

 selected. 



For clearing or dealcoholizing objects intended for embedding in wax, it is 

 customary to use a hydrocarbon, and xylene is in almost universal employ- 

 ment at the present time. From the point of view of physical properties, there 

 is little choice between benzene, xylene, and toluene; the author considers that 

 the first of these has a distinct advantage in that it tends to render objects 

 less brittle. All three are equally miscible with molten paraffin, but none is as 

 good a solvent of solid paraffin as is chloroform. Both the hydrocarbons and 

 chloroform are very sensitive to water, so that it is essential that an object be 

 completely dehydrated in alcohol or dioxane before being transferred to one 

 of these clearing agents. It occasionally happens that it is impossible to pro- 

 vide perfect dehydration, and then one is forced to utilize the "coupling" 

 properties of phenol, which is usually employed in the following mixture: 



Carbolxylene: 

 Xylene 70 ml. 

 Phenol 30 Gm. 



