70 BASIC METHODS FOR EXPERIMENTS 



mentioned above. The chief difficulty to be overcome is that 

 due to the presence of moisture. For this reason I am very 

 careful with the absolute alcohol that I use. I keep it in glass 

 stoppered bottles of not more than 250 cc. capacity sealed with 

 wax or paraffin. 95 per cent and absolute alcohol take up 

 moisture very quickly at the sea-side. The xylene or toluene is 

 not so hydroscopic. The great danger comes not from the 

 xylene itself taking up the moisture but from the fact that the 

 absolute alcohol contains moisture and this is carried over when 

 the eggs are transferred from the absolute alcohol to the xylene 

 or toluene. 



Preparation of the Paraffin Block 



A moment after the last paraffin has been added to the eggs, 

 the dish is carefully removed from the hot-plate and gently 

 lowered into a finger bowl containing 80 per cent alcohol and 

 surrounded by one part cracked ice and two parts water in a 

 dish of about 2 liters capacity. At first only the bottom of the 

 crystal containing the eggs is immersed in the cooled alcohol. 

 Slowly more of the crystal is lowered until a solidifying film 

 appears on the upper surface of the paraffin. Then the crystal 

 is carefully tilted to allow alcohol to run over the hardening 

 surface without breaking the newly formed film or even making 

 a ripple on it. Now the crystal is cautiously released and fully 

 submerged. The alcohol is next covered with a glass plate. 

 In a few minutes the block of paraffin becomes dislocated and 

 floats. If it does not separate from the crystal, pressure of the 

 linger on the rim of the block may dislodge it. Should it still 

 not come away, cut around the rim of the paraffin with the point 

 of a knife and gently press on the edge with the fingernail. The 

 failure now to dislodge the paraffin block means improper 

 infiltration of the paraffin, presence of xylene or of alcohol: the 

 paraffin may come away leaving all or some of the eggs adhering 

 to the glass. 



If one immerses the congealing paraffin mass in a too cold 

 medium, as ice and water or cold running water, too rapid cooling 

 of the mass obtains with the result that the paraffin block cracks. 

 The presence of these fissures is an obstacle to good sectioning. 



