O CHAPTER I. 



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. 



This is a convenient stage for carrying out inhiulr 

 dissections, if any such have to be done, a drop of clearing 

 agent being a most helpful medium for carrying out such 

 dissections (see 9). 



At this point the course of treatment follows one of two 

 different roads, according as the object is to be mounted 

 direct in balsam ( 8), or is first to be sectioned ( 0). 



6. Imbedding, and Treatment of Sections. The objects are 

 now imledded. They are removed from the clearing medium, 

 and soaked until thoroughly saturated in the imbedding 

 medium. This is, for small objects, generally paraffin, 

 liquefied by heat, and for large objects either paraffin or a 

 solution of collodion or " celloidin " (in this latter case the 

 clearing may be omitted and the tissues be imbedded direct 

 from the alcohol). The imbedding medium containing the 

 object is then made to solidify, and sections are made with a 

 microtome through the imbedding mass and the included 

 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 

 damar. Or they may be stained, washed, dehydrated, and 

 cleared in watch-glasses, and afterwards mounted as desired 

 the imbedding medium being first removed if desirable. 



Or, the material may be stained in bulk, before cutting the 

 sections. In this case the object, after ha.ving been fixed 

 and washed out, is taken from the water, or while still on its 

 way through the lower alcohols (it should not be allowed to 



