76 CYTOLOGICAL TECHNIQUE 



molecules cannot enter freely. As a result the embedded tissue, 

 being dry, friable, and not properly supported, often cracks or 

 crumbles during sectioning. 



When replacement of the antemedium is complete, the paraffin 

 is hardened by crystallization on cooling. Each long molecule 

 places itself parallel with its neighbours. The molecules are not 

 fully extended, for each forms a zigzag in one plane. ^^^ Crystal- 

 lization occurs readily because the molecules, unlike those of 

 certain other waxy constituents of petroleum, have no major 

 branches that would interfere with close packing. Not every 

 molecule has a branch ; such branches as occur arise near one end 

 of the molecule, and are usually only one carbon atom long.^"^^ 



The crystals take the form of plates or needles. Each molecule 

 is held in position only by van der Waals forces, and this accounts 

 for the softness of the material and its low melting-point. There 

 is a tendency to a lamellar structure in the crystal, for all the 

 molecular zigzags lie in parallel planes, and each molecule in each 

 monomolecular plane is somewhat more strongly attracted to its 

 neighbours in that plane than to the molecules of other planes. ^^^ 

 It is presumably the slipping of the planes on one another that 

 gives paraffin waxes their greasy feel. 



It is generally believed that paraffin blocks should be cooled 

 quickly, by plunging them into cold water. The wax then hardens 

 in the form of small crystals, and these are supposed to give 

 better cutting qualities than large crystals. An experiment was 

 performed to test this belief. The kidney of a mouse was cut in 

 two. Both the pieces were fixed in Zenker, dehydrated in ethanol, 

 and passed through toluene into melted wax. One piece of 

 wax was cooled suddenly, in the usual way. The cooling of the 

 other was done in stages: first an hour at 45° C, then an hour at 

 37° C, then further cooling in a warm room (22° C). Both pieces 

 were cut on the same microtome and the sections dyed and 

 mounted. The two blocks cut equally well and the final prepara- 

 tions were indistinguishable.^^ 



Sections of tissue that have been thoroughly permeated by 

 paraffin may be freely exposed to the air without risk of damage. 

 This makes it easy to stick them to glass slides, and as a result 



