INTRODUCTORY. 6 



is not necessary, may be dehydrated much quicker than this. 

 Infusoria may be prepared in a few minutes.) 



The water having been thus completely removed, the 

 alcohol is in its turn removed from the tissues, and its place 

 taken by some anhydrous substance, generally an essential 

 oil, which is miscible with the material used for imbedding. 

 This operation is known as Clearing. It is very important 

 that the passage from the last alcohol to the clearing agent be 

 made gradual. This is effected by placing the clearing medium 

 under the alcohol. A sufficient quantity of alcohol is placed 

 in a tube (a watch-glass will do, but tubes are generally 

 better), and then with a pipette a sufficient quantity of clearing 

 medium is introduced at the bottom of the alcohol. Or you 

 may first put the clearing medium into the tube, and then care- 

 fully pour the alcohol on to the top of it. The two fluids 

 mingle but slowly. The objects to be cleared being now 

 quietly put into the supernatant alcohol, float at the surface of 

 separation of the two fluids, the exchange of fluids takes place 

 gradually, and the objects slowly sink down into the lower 

 layer. When they have sunk to the bottom, the alcohol may 

 be drawn off with a pipette, and the objects will be found to 

 be completely penetrated by the clearing medium. (It may 

 be noted here that this method of making the passage from 

 one fluid to another applies to all cases in which objects have 

 to be transferred from a lighter to a denser fluid for instance, 

 from alcohol, or from water, to glycerin. It is a more exact 

 method than that of successive baths of mixture of alcohol 

 and clearing agent.) 



The objects are now imbedded. They are removed from 

 the clearing medium, and soaked until thoroughly penetrated 

 in the imbedding medium. This is, for small objects, gene- 

 rally paraffin, liquefied by heat, and for large objects generally 

 a solution of collodion or " celloidin." The imbedding medium 

 containing the object is then made to solidify, as described in 

 the chapter on imbedding processes, and sections are made 

 with a microtome through the imbedding mass and the in- 

 cluded objects. The sections are then mounted on a slide by 

 one of the methods described in the chapter on Serial Section 

 Methods, the imbedding material is removed from them (in 

 the case of paraffin), they are stained in situ on the slide, 

 dehydrated with alcohol, cleared, and mounted in balsam or 



