THE PARAFFIN METHOD 



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Fig. 21 



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. 

 Or, keeping the imbedding tray at the temperature of the paraffin 

 bath — never hotter — the objects may be picked out gently and arranged 

 as they are placed in the 

 paraffin in the tray. Look 

 at Figures 21 and 22, repre- 

 senting the arrangement of 

 root-tips in a paraffin cake. 

 From a cake like that in 

 Figure 21 it is easy to cut 

 out tips for sectioning. The 

 arrangement, or rather the 

 lack of it, shown in Figure 22 

 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 suf- 

 ficiently 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 min- 

 utes, 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 par- 

 affin 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 Silphiu?n, 

 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. 



Fig. 22 



Figs. 21 and 22. — Paraffin cakes of root tips, the upper 

 (Fig. 21) showing a good arrangement; the lower (Fig. 

 22) showing fewer tips and most of these not in position 

 to be blocked without injury. 



