THE CELLOIDIN METHOD 137 



few chips of dry celloidin from time to time until the mixture is quite 

 stiff and firm. Remove each block with the celloidin adhering to it and 

 harden it in chloroform for 12 hours. Then transfer to a mixture of 

 equal parts of glycerin and 95 per cent alcohol, where the material 

 should remain for a few days before cutting. 



Cutting, staining, and mounting. — Although 10 m is usually thin 

 enough, sections are readily cut as thin as 5 m by this method. Re- 

 move the celloidin before staining by treating 10-15 minutes with 

 ether; then wash in 95 per cent alcohol and transfer to water, and 

 then to the stain. Stain to a fairly dense purple in an aqueous solution 

 of Erlich's haematoxyhn; wash in dilute aqueous solution of calcium 

 or sodium carbonate, and then in two changes of distilled water. Add 

 a few drops of alcohohc solution of equal parts of Griibler's alcoholic 

 and aqueous safranin, and stain to a rich red. A dilute stain acting 

 1-2 hours is better than a more concentrated stain acting for a shorter 

 time. Transfer directly to absolute alcohol, clear in xylol, and mount 

 in balsam. 



Haidenhain's iron-haematoxylin is a very satisfactory stain for 

 photographic purposes. 



The celloidin method has its disadvantages as well as its advan- 

 tages. It is extremely slow and tedious, and it is rarely possible to cut 

 sections thinner than 10 /z while, on the other hand, it gives smoother 

 sections. 



Succulent tissues, which are usually damaged by the paraffin meth- 

 od, are easily handled without any injury in celloidin. The fact that 

 the method may be used without heat is often a further advantage. 

 Stems and roots, which cannot be handled at all in paraffin, cut well 

 in celloidin, and much larger sections can be cut than in paraflan, but 

 most material of this kind can be cut without any imbedding. 



When material is to be imbedded, use celloidin as a last resort. Use 

 parafiin when you can, celloidin when you must. 



