200 COLLODION AND OTHER IMBEDDING METHODS. 



303. Cutting. If the object has not been stained before 

 imbedding, it will 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 bergamot oil used for clearing. 



To fix a collodion block to the microtome, proceed as fol- 

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

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

 microtome. Cover it with a layer of collodion, which you 

 allow to dry. Take the block of collodion, 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 prepared wood or pith, 

 and press down tightly on to it the wetted or dried surface 

 of the block of collodion. Then throw the whole into weak 

 (70 per cent.) alcohol for a few hours (or even less), or into 

 chloroform for a few minutes, in order that the joint may 

 harden. 



Dr. LINDSAY JOHNSON informs me that he finds it very 

 convenient to take for this purpose the cement used by metal 

 turners for fastening metal objects on to boxwood chucks. 

 The exact composition of this cement varies somewhat, but 

 an average one is beeswax, 1 part; rosin, 2 parts. To use 

 it, you must get the block of celloidin perfectly dry at the 

 bottom, then warm the object- holder slightly, if possible, 

 over a flame; drop on to it a few drops of melted cement, 

 and press on to it the block of celloidin, which will be firmly 

 fixed as soon as the cement is cool that is, in a few 

 seconds. 



For objects of any considerable size it is important not to 

 use cork for mounting on the microtome, especially if the 

 object-holder be a vice ; for cork bends under the pressure of 

 the holder, and the elastic collodion bends with it, deforming 

 the object. I have seen large embryos so deformed in this 

 way that the sections obtained were true calottes, segments of 

 a sphere. If the object-holder be of the cylinder type, as in 

 the later forms of the Thoma microtome, the above-described 



