FREEZING-DRYING PREPARATION OF TISSUE 9 



can be applied here since the time for total dehydration is a function 

 of a number of poorly defined variables. 



6. When dehydration is considered complete, lower away the 

 Dewar cooling flask and allow the tissue to come to room tempera- 

 ture. 



7. Start the paraffin heater and keep the temperature of the 

 paraffin just above its melting point. The top of the paraffin will 

 melt first; regulate the heating so that a portion of solid paraffin 

 remains at the bottom of the chamber. Conditions of — 66° and 

 4 X 10"^ mm. of mercury are obtained routinely during operation. 

 The tissue will sink into the melted paraffin without causing a single 

 bubble to rise if the tissue is properly dehydrated and the paraffin 

 completely degassed; otherwise bubbling will occur. 



8. Break the vacuum after infiltration is complete and the par- 

 affin has been allowed to solidify; remove and block for cutting. 



