EMBEDDING 73 



each piece of tissue and does not divulge which is which, it will 

 be found difficult or impossible to guess. 



If the fixative contained mercuric chloride, iodine should be 

 dissolved at 0-5% w/v in one of the grades of ethanol (for 

 instance, 80%), to prevent the formation of a black deposit 

 (p. 36). 



If the fixative contained no water (Clarke, p. 59, for instance), 

 the tissue should simply be washed in absolute ethanol. 



Although the tissue has now been dehydrated, it cannot be 

 invaded by melted paraffin wax, because ethanol and paraffin 

 are immiscible. It is necessary that the tissue should pass through 

 a fluid that is freely miscible with both. Antemedia^ are fluids in 

 which tissues are soaked immediately before they are transferred 

 to embedding media. Some of them have about the same re- 

 fractive index as dehydrated protein. Since they fill up the spaces 

 in the tissue not occupied by dehydrated protein, they render it 

 more or less optically homogeneous and transparent. They are 

 therefore often called 'clearing' agents, but the capacity to 'clear' 

 is a useless attribute of certain antemedia, and some of the best 

 do not possess it. A large number of more or less harmless fluids 

 are miscible with both ethanol and melted paraffin, and one may 

 choose one's antemedium from among them. Toluene is as good 

 as any for routine use. This light, colourless liquid is so called 

 because it was first obtained by distillation of an oleo-resin 



CH. 



Toluene 



exported from Tolu in Colombia; but nowadays it is got from 

 coal tar. Its boiling-point (1 10° C) is well above the melting-point 

 of paraffin wax. Tissues tend to be distorted (unevenly shrunk) 

 when passed directly from ethanol to toluene, and it is therefore 

 better to pass them instead to a mixture of ethanol and toluene in 

 equal volumes, and from this to pure toluene. 



Ethylene glycol mono-ethyl ether ('cellosolve') may be used 

 instead of ethanol as a dehydrating agent. The relationship of 



