THE PARAFFIN METHOD 
115 
not only injures the material, but also prevents a thorough imbedding. 
Pour the paraffin with the objects into the imbedding-dish and arrange 
them so as to facilitate the future cutting-out from the paraffin cake. 
Look at Figures 22 and 23, representing the arrangement of root-tips 
in a paraffin cake. From a cake like that in Figure 22 it is easy to cut 
out tips for sectioning. The 
arrangement, or rather the 
lack of it, shown in Figure 23 
should be remembered only 
as an exasperating example. 
After the objects have 
been arranged, cool the cake 
rapidly by allowing the bot¬ 
tom of the dish to rest upon 
cold water. As soon as a 
sufficiently firm film forms 
on the surface of the cake, 
let water flow gently over 
the top. After the cake has 
been under water for a few 
minutes, the paraffin will 
either come out and float 
on the water or, at least, it 
will be easily removed from 
the dish. If paraffin cools 
slowly it crystallizes and 
does not cut well. The layer 
of paraffin should be just 
thick enough to cover the 
objects, not only as a matter 
of economy, but because a thick layer retards the cooling. Very 
small objects, like the megaspores of Marsilea, ovules of Silphium, 
etc., may simply be poured out upon a cool piece of glass, which has 
been smeared with glycerin or soapy water. In this way, thin cakes 
are made which harden very rapidly. 
If one is doing much imbedding, it is worth while to have the par¬ 
affin cakes uniform in size and to have a convenient method of filing. 
In our own collection, there are more than 6,000 paraffin cakes. They 
are filed in pasteboard boxes 28 cm. long, 10 cm. wide, and 2 cm. deep. 
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Fig. 22 
Fig. 23 
Figs. 22, 23. —Paraffin cakes of root-tips, the upper 
(Fig. 22) showing a good arrangement, the lower (Fig. 23) 
showing fewer tips and most of these not in position to 
be blocked without injury. 
