734 ' HISTOLOGIC TECHNIC 



general use the procedure recommended by Gage ("The Microscope") 

 is found to be very satisfactory. The tissues after fixation are suc- 

 cessively placed for one or two days in each of the following strengths 

 of alcohol 70, 80 and 95 per cent. They are then returned to 80 per 

 cent, alcohol, where they may remain indefinitely, but it is generally 

 safer to embed for sectioning without great delay; this is especially 

 true of tissue which has been fixed with Zenker's solution. Most fixing 

 fluids also harden; the terms 'fixing* and 'hardening' are consequently 

 often used synonymously. 



EMBEDDING 



Thick sections may be obtained from the firmer tissues by freehand 

 sectioning with a razor, but for the satisfactory preparation of thin sec- 

 tions a microtome is a necessity and the tissues must have been pre- 

 viously embedded to render them sufficiently firm. This is accomplished 

 by infiltrating the tissue with celloidin or paraffin, either of which yields 

 a firm, waxy consistence. 



Embedding in Celloidin. Make a saturated solution of a little cel- 

 loidin (Schering's) in a mixture of equal parts of alcohol and ether. 

 The alcohol should contain no trace of copper sulphate. This solution 

 is for convenience known as number III and should have a very thick, 

 syrupy consistence. 



A small portion of number III is mixed with three to five times its 

 volume of the alcohol and ether mixture, to obtain number II, which 

 should have a somewhat viscid consistence. 



A second small portion of number III is diluted with ten to fifteen 

 times its volume of the alcohol and ether, to produce celloidin number 

 I, which should have a thin, watery consistence. 



Small pieces of tissue which have been thoroughly hardened in 95 

 per cent, alcohol are treated as follows : 



1. Dehydrate in absolute alcohol, six to twenty-four hours. 



2. Place in the absolute alcohol and ether mixture, twelve to twenty- 

 four hours. 



3. Place in celloidin number I, twelve to twenty-four hours. 



4. Place in celloidin number II, twelve to twenty-four hours. 



5. Place in celloidin number III, twenty-four to forty-eight hours, 

 or longer. 



Pieces of tissue of considerable size may be satisfactorily embedded 



