12 



Paraffin Sections 



General Principles 



Nature of the Process 



The last chapter discussed freehand sec- 

 tions, that is, sections of material which is 

 in itself sufficiently strong and sufficient!}' 

 coherent to hold together when cut in thin 

 slices. The majority of objects to be sec- 

 tioned, however, contain cavities which 

 would collapse under the action of the 

 knife, or are not of a shape or consistency 

 which would enable one of them to be cut 

 by hand. Objects of this nature must, 

 therefore, be supported in a matrix which 

 ■will itself section well, and those contain- 

 ing cavities must be impregnated through- 

 out their whole substance with the embed- 

 ding medium. Wax, nitrocellulose, and a 

 variety of water-soluble materials have 

 from time to time been suggested as im- 

 pregnating and supporting agents, but the 

 use of wax is so convenient and simple 

 that only in special cases should any other 

 material be emploj'ed. 



The advantage of wax is not only that 

 it readily passes from a solid to a molten 

 state at temperatures which do not dam- 

 age the material, but also that it is some- 

 what sticky, so that ribbons of sections 

 may be prepared, each section being in the 

 ribbon in the same order as it was cut from 

 the object. Thus, if a rectangular block of 

 wax is mounted in some kind of holder and 

 then brought sharply down on a hori- 

 zontal knife, the thin slice of wax which is 

 cut off will adhere by its edge to the edge 

 of the knife. If the block is then advanced 

 by some mechanical device — such as a 

 microtome — a small distance and again 

 brought down on the knife, a second sec- 

 tion will be cut off which will displace the 

 first section, to which it will adhere on one 



94 



edge, while the other edge remains at- 

 tached to the knife. By the repetition of 

 these movements a long ribbon may be 

 produced. A ribbon of this type is seen in 

 all the stages of its preparation in Figs. 

 65-70. Preparation of paraffin sections is 

 quite a complex operation and in\'olves 

 the following stages: 



1. Fixation of the material. 



2. Dehydration, in order that the ma- 

 terial may be impregnated with a 

 fluid capable of dissolving wax. 



3. The removal of the dehydrating 

 agent with a material solvent of, or 

 miscible with, molten wax. 



4. The soaking of the cleared specimen 

 in a molten wax for sufficiently long 

 to insure that it shall become com- 

 pletely impregnated. 



5. Casting the now impregnated speci- 

 men into a rectangular block of wax. 



6. Attaching this block of wax to some 

 holder which itself may be inserted 

 into a suitable microtome. 



7. The actual cutting of the sections 

 of the block into ribbons. 



8. The placing of these ribbons on a 

 glass shde in such a manner that 

 they will lie flat and that the con- 

 tained section will be adherent after 

 the wax has been dissolved away. 



9. The removal of the wax solvent. 

 10. Staining and mounting. 



I^ach of these operations will be dealt with 

 in due order. This chapter terminates with 

 a series of examples which describe in de- 

 tail the application of the principles dis- 

 cussed to actual preparations. 



