EMBEDDING 83 



its single electron and attaches itself to the growing point of 

 another molecule (which is thus terminated and becomes 'dead'). 

 The carbon atom now has an uneven number of electrons, and 

 therefore acts as a new growing point, to which other monomers 

 add themselves one after another. The branch thus formed may 



CH3 

 H I 

 — C— C— 



H I .0 

 Cf H 



\OC(CH,)2CH3 



A segmer of butyl methacrylate, with an active carbon 

 (C) in the butyl radicle 



itself be terminated and 'die' as a loose end; but if it meets another 

 growing branch, the two may fuse and thus accomplish a union 

 between two branched molecules that were previously separate. 

 Since the branches are not all formed in the same plane, a three- 

 dimensional meshwork results. 



Tissues shrink slightly in butyl methacrylate while the latter is 

 still in monomeric form.^ Polymerization involves further shrink- 

 age, since the methacrylate itself becomes reduced in volume. 

 The various methacrylate esters differ from one another in this 

 respect. The longer the molecule of the esterifying alcohol, the 

 less the reduction in volume on polymerizing. Butyl methacrylate 

 contracts to about 85% of its former volume, methyl methacry- 

 late to about 79 %.^^^ The butyl ester is therefore preferable unless 

 a very hard block is required. Measurements of the volume of the 

 embedded tissue'' suggest that this shrinks more than the metha- 

 crylate. Shrinkage is less than with paraffin, and since it is nearly 

 equal in all directions, there is scarcely any distortion. 



Blocks containing tissue can be stored indefinitely in the dry 

 condition. To obtain sections from 8 /< to 1 /^ thick, it is best to 

 soak the block overnight in 70 % ethanol and to cut on a sliding 

 microtome with an oblique knife flooded with 70% ethanol. ^^ 

 The back of the knife should be raised rather higher above the 

 level of the cutting edge than is usual when cutting paraffin. 

 Sections may be stored in 70% ethanol. 



