6. ^Aicrotome Sectioning of Material in Paraffin 



Material embedded in paraffin is almost invariably cut with a 

 rotary microtome, in which the knife is stationary and the piece of 

 tissue is moved up and down past the cutting edge. Cutting is ac- 

 complished by wedge action, like the action of a chisel or a plane. 

 After a section has been cut, and the tissue carrier has passed the 

 knife on the upstroke, an automatic feed mechanism advances the 

 tissue carrier forward, and another section is cut. Successi\c slices 

 remain attached to each other, forming a ribbon of paraffin. Ihe 

 successive sections cut from a piece of tissue are thus kept in serial 

 order, and this older can be j^reserved throughout the processing of 

 the slides. From serial slices of an organ of a plant it is possible 

 to reconstruct the external or internal structure of the organ, of a 

 tissue system, or even of a single cell. As an example of serial section- 

 ing we may use the homely illustration of a loaf of bread, cut into 

 slices and the slices hiid out in order. 



The piece of material to be sectioned is fastened to a mounting 

 l)lock, which is clamped into the microtome. Inexpensi\e mounting 

 blocks can be made of hard, porous wood, such as oak or ash. 1 he 

 most useful sizes range from I by 1 by 2 cm. to 2 b) 2 by 3 cm. Soak 

 the blocks in hot canning wax. Bakelite and other |ilastics make 

 excellent l:)locks, preferred to wood l)(.cause j)lastic blocks do not 

 comjjress when clamped into the miciotonu" (Fig. ().l A). Wood or 

 plastic blocks arc satisfactory for most woik. being inexpensive and 

 sufficiently rigid for sections over (I \y in thickness. For loutine sam- 

 pling of material, many pieces of tissue can be mounted on separate 

 blocks, the mounting l)Iocks miml)ered by means of string tags, and 

 test sections made from each piece. Hie mounied blocks can be kept 

 with the j)roj)er batches of embedded material until staining trials 

 establish which block has the desired siaye. 



[40] 



