272 TECHNIQUE 



the paraffin hardens thronghont homogeneously and quickly. In 

 a few minutes the hardened paraffin block can be removed and stored 

 indefinitely in this form or prepared for sectioning. If the weather 

 is very warm, or if the technique is carried out in a warm room, one 

 would use a higher melting-point paraffin, while a lower melting- 

 point ])araffin is suitable for use in winter or under colder conditions. 

 If very thin sections are required, one must use a harfler (higher 

 melting-point) paraffin than if thicker sections were to be made. 

 Ordinarily it is wise to avoid heating the tissue any higher than 

 absolutely necessary in order to avoid the hardening and shrinking 

 effect of heat, so that 58° C. represents a maximum temperature 

 for the paraffin bath. 



In the foregoing account of dehydration we passed from 95 per 

 cent alcohol to 100 per cent alcohol and from this to xylol. Since 

 100 per cent alcohol is expensive, and since it tends to harden tissues, 

 it can be dispensed with by the use of aniline oil. This is cheaper 

 and does not harden or shrink the tissues. The following tables 

 exi^lain the two procedures: It is understood that the tissue has 

 been properly fixed, washed free of fixative, and has been stored in 

 70 per cent alcohol. 



Procedure. 



Absolute Alcohol Method. Aniline Oil Method. 



80 per cent alcohol, one hour. §—80 per cent + i aniline, one hour. 



95 per cent alcohol, one hour. 3—95 per cent + f aniline oil, one hour. 



100 per cent alcohol, one hour. Aniline oil (until translucent), one hour. 



^ — 100 per cent + J xylol, one hour. | aniline + i xylol, one hour. 



Xylol, one hour. Xylol, one hour. 



Fresh xylol, one hour. Fresh xylol, one hour. 



I xylol + 5 melted paraffin, one hour and kept in warm place so that the paraffin 



remains melted. 

 Pure melted paraffin (about 52° C. m.p.) in the paraffin oven, one hour. 

 Change the paraffin and keep specimen in oven, one hour. 

 Change the paraffin again and keep specimen in oven for about one hour. 

 Embed as directed above. 



Toluol is an excellent substitute for xylol and tissues can be left in it for longer 



periods. Furthermore, it is more volatile and therefore more readily lost during 



paraffin transfers. 



CELLOIDIN EMBEDDING. 



For material which is tougher, or which demands a technique 

 which does not involve heating, a slower method of embedding in 

 celloidin instead of paraffin has been devised. It has the advan- 

 tage of causing less shrinkage and distortion but does not permit 

 such thin sections to be easily cut. Only single sections are made 

 at a time, and so the handling of large numbers of sections is more 



