ZOOLOGY AND BOTANY, MICEOSCOPY, ETC. 569 



Cutting. — There are two forms of microtome suitable for cutting 

 sections in series. In one the tissue is raised directly by a fine screw, 

 and the sections cut with an ordinary razor : in the other form, now 

 so largely used, the tissue in its holder is moved up an inclined 

 plane, aud the sections cut with a large knife which works backwards 

 and forwards in a horizontal slot running parallel with the inclined 

 plane. In both cases the machine is fixed to the table or heavy 

 enough to remain steady, so that while the razor is worked with one 

 hand the other hand is at liberty to hold a little paper spatula — a 

 small piece of paper run on at the end of a small scalpel — to prevent 

 the sections curling. The paraffin block is pared down to the smallest 

 size possible, and, as the razor is drawn along, the edge, which com- 

 mences to curl, is caught by the paper and prevented from so 

 doing ; the section is then transferred to the slide. 



Preparation of the Slide. — The slide is smeared with a strong 

 solution of shellac in anhydrous creosote. Care must be taken to 

 have as little as possible on the slide. By this method the sections 

 are stuck to the slide, thereby saving the most delicate objects from 

 falling to pieces after the paraffin is removed, and enabling one to 

 mount numerous sections on one slide. The importance and value of 

 this treatment cannot be over-estimated. It enables one to mount 

 with absolute certainty whole sections of the most friable objects, 

 such as an insect, without a single fragment of the section becoming 

 displaced. 



Mounting. — The slide bearing the sections is now placed in a 

 water-oven or on a tin box containing water at a temperature two or 

 three degrees above the melting point of the paraffin used. The 

 slide is left here for at least half an hour. The object of this warming 

 is twofold, to evaporate the creosote and to melt the paraffin. 



The slide is now taken up, and while the paraffin is still molten 

 is flooded with turpentine dropped from a small pipette. This dis- 

 solves melted paraffin instantaneously, and precipitates the shellac 

 fastening the sections to the slide. The turpentine is allowed to 

 flow off. and replaced by new until all the paraffin is removed. The 

 slide is then allowed to drain, the edges wiped, and the cover-glass 

 put on. The Canada balsam, which should be very fluid, is placed on 

 the under surface of the cover-glass ; this is turned over and quickly 

 lowered. The balsam dissolves the shellac, and if the cover- glass is 

 not put on very quickly the sections may shift or delicate sections 

 come to pieces and float off the slide. It being necessary to use the 

 balsam in such a fluid condition, and a certain amount of turpentine 

 always remaining upon the slide, the slides should be looked over the 

 next day and more balsam added at the edge of the cover-glass if 

 necessary. These methods, especially that of fastening the sections 

 to the slide with shellac, although suggested and elaborated by zoo- 

 logists for the purpose of mounting serial preparations, will, no 

 doubt, come into very general use in ordinary histology for such 

 tissues as placenta or spleen where very thin sections have always 

 been found liable to fall to pieces, and the most important pieces to 

 fall out and be lost. 



