6 CHAPTER I. 



For material that is intended only for section- cutting, 1 

 find that by far the best plan is to clear and imbed at once 

 in paraffin. This affords, as far as I can see, an absolutely 

 perfect preservation. I have worked on material that has 

 been preserved in this way for over seven years. The pre- 

 servation of the tissues, down to the finest details of cell- 

 structure, appears to be perfect, and the staining as precise 

 as when the specimens were first put up. The only notice- 

 able defect is that the tissues are rather brittle, and do not 

 Gut well ; but it is not certain that that is not owing to their 

 having been over-hardened in the first instance. Cedar- wood 

 oil is, I find, nearly, if not quite, as good as paraffin. 



4. Removal of Alcohol ; Clearing, The water having been 

 thus sufficiently 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 or mounting. This opera- 

 tion is generally 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 carefully 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 

 after some further lapse of time the objects will be found to 

 be completely penetrated by the clearing medium. 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 should be noted here that this is the proper stage for 

 carrying out minute dissections, if any such have to be done, 



