128 CHAPTEB IX. 



degree of hardening of the mass ; but I doubt whether it is possible to carry 

 the hardening much beyond the point attained by the chloroform or alcohol 

 method without incurring a very undesirable degree of shrinkage. 



The hardening processes used in the method of clearing 

 before cutting, which I prefer to all the foregoing, will be 

 described later on, 165. 



158. Preservation. The hardened blocks of collodion may 

 be preserved till wanted in weak alcohol (70 per cent.). 

 They may also be preserved dry by dipping them into melted 

 paraffin (APATHY, Zeit. f. wiss. Mik., v, 1, 1888, p. 45), or, 

 after rinsing in water, in glycerin-jelly, which may be re- 

 moved with warm water before cutting (APATHY, Mitth. Zool. 

 Stat. Neapel, xii, 1897, p. 372). 



Reference numbers may be written with a soft lead pencil 

 on the bottom of the paper trays, or with a yellow oil pencil 

 on the bottom of the watch glasses in which the objects are 

 imbedded. On removal of the paper from the collodion 

 after hardening, the numbers will be found impressed on the 

 collodion. 



159. Cutting. If the object has not been stained before imbedding, it 

 may form so transparent a mass with the collodion that the arrangement of 

 the object and sections in the right position may be rendered very difficult. 

 It is, therefore, well to stain the collodion lightly, just enough to make its 

 outlines visible in the sections. This may be done by adding picric acid or 

 other suitable colouring matter dissolved in alcohol to the collodion used for 

 imbedding, or to the oil used for clearing. 



To fix a collodion block to the microtome proceed as 

 follows. Take a piece of soft wood, or, for very small 

 objects, pith, of a size and shape adapted to fit the holder 

 of the microtomy. Cover it with a layer of collodion, which 

 you allow to dry. Take the block of collodion or the in- 

 filtrated and hardened but not imbedded object, and cut a 

 slice off the bottom, so as to get a clean surface. Wet this 

 surface first with absolute alcohol, then with ether (or allow 

 it to dry); place one drop of very thick collodion on the pre- 

 pared wood or pith and press down tightly on to it the wetted 

 or dried surface of the block or object. Then throw the 

 whole into weak (70 per cent.) alcohol for a few hours, or 



