132 CYTOLOGICAL TECHNIQUE 



phenyl group for the carboxyl of the acid. The polymer that is 

 used in DPX has rather less than 800 segmers in most of its 



A segmer of acrylic acid A segmer of polystyrene 



molecules. It is known by the trade name of 'distrene 80'. It is 

 a colourless solid, freely soluble in xylene. 



If a section or other object be mounted in a solution of distrene 

 80 in xylene, the polymer retracts under the edges of the coverslip 

 as the solvent evaporates. This defect is mitigated or overcome by 

 the addition of a plasticizer to the medium. Typical plasticizers 

 are non-volatile solvents of the plastic on which they act. They 

 are supposed to associate themselves with reactive groups in the 

 polymer, which would otherwise attach themselves to other re- 

 active groups, and thus link the long molecules into a rigid mesh- 

 work ; but they do not combine with the polymer so as to form 

 a new substance. Thus they make the polymer softer than it would 

 otherwise be, and oppose its contraction when any volatile solvent 

 evaporates. The plasticizer in DPX is tri-/7-tolyl phosphate (often 

 called tricresyl phosphate) ; that is to say, an ester of phosphoric 

 acid, in which three/?-tolyl radicles have replaced three hydrogens. 



CH, 



The p-tolyl radicle 



This plasticizer is a colourless, non- volatile liquid of high refrac- 

 tive index (about 1-56). 



To prepare DPX, add to 100 ml of xylene 18-75 ml of tri-/?-tolyl 

 phosphate, and then 25 g of distrene 80. The latter is obtainable from 

 Messrs Honeywill and Stein, 21 St James's Square, London, S.W.I. 

 The plasticizer is irritating to the skin and contact should be avoided. 



