MAKING SECTIONS 57 



The Technique of Dehydrating, Clearing, and Embedding. Before pass- 

 ing to the choice of a microtome and the method of using it, it is necessary 

 to discuss briefly the actual operations which are involved in using the dehy- 

 drating, clearing, and embedding media selected. The techniques of dehydration 

 and dealcoholization do not differ materially from those used in the preparation 

 of wholemounts which have been described. The whole process, however, 

 could be much simplified if people would only remember that water is heavier 

 than the majority of dehydrating agents and that the majority of dehydrating 

 agents are lighter than most clearing agents. In translating this theory into 

 practice, it must be obvious that the object to be dehydrated should be sus- 

 pended toward the top of a tall cylinder of dehydrant, in order that the water 

 extracted from it may fall toward the bottom of the vessel, and that an object 

 to be cleared should be held at the bottom of the vessel for the reverse reason. 

 It is, indeed, practically impossible to dehydrate a large object unless it is so 

 suspended. The process of impregnating the tissues with wax has not been 

 discussed previously and will be dealt with Rally. 



It is obvious that the first prerequisite is some device which will maintain 

 the temperature of the wax at just above its melting point. Most people 

 employ complex thermostatically controlled ovens for this purpose, but the 

 exceedingly simple device shown in Fig. 20 is practical and cheap. As will be 

 seen, this device consists essentially of a series of incandescent electric bulbs 

 held at a distance which may be varied above a series of glass vials. Before 

 the embedding process is started, as many vials as will be required are filled 

 with wax, placed under the reflector, and the current is turned on. After a 

 little while, it will be observed that the absorbed heat has melted the wax. 

 The wax may be melted only at a small surface layer, throughout the entire 

 vial, or, as is required, in the upper two thirds of the vial. If this last is not 

 achieved, the height of the lamp must be varied until after an hour or two 

 each of the vials contains about one third of unmolten opaque wax at the bot- 

 tom and two thirds of the clear molten material above. Thus, when the object 

 is placed in one of these vials, it will drop until it reaches the solidified layer 

 where it will remain in contact with molten wax at exactly the melting point 

 of the wax. It is obvious that the room in which this operation is to be con- 

 ducted must be at a fairly constant temperature and must be relatively free 

 of drafts, but only a very large volume of embedding work justifies the pur- 

 chase of an expensive thermostatically controlled oven. If such an oven is to 

 be purchased, it is highly desirable to avoid one in which the heat is distrib- 

 uted by convection. The oven shown in Fig. 21, in which a circulating fan 

 continuously moves the air and thus maintains a uniform temperature through- 



