APPENDIX. 



viously stained or not ; the object of the manipulation is to replace 

 the support afforded by the paraffin by attaching the sections to the 

 slide before removing the embedding substance. In this point paraf- 

 fin is much more accurately and conveniently worked than celloidin, 

 since the latter is removed from the sections with much less facility 

 than paraffin. The ideal fixing solution is yet to be devised, those 

 at present employed being all defective in some particular. The 

 desiderata are secure attachment of the sections to the slide in all 

 solutions necessary for the various manipulations of staining and 

 mounting, and complete expansion of the sections before their final 

 adhesion to the slide : this latter consideration is of great importance 

 in large sections or in mounting ribbon-series, since it is practically 

 impossible to cut these without some slight compression or wrinkling; 

 if mounted without being perfectly expanded, the preparations are 

 marred by distorting folds, which in lines of accurate work, where 

 reconstructions are sometimes necessary, are serious defects. The 

 most satisfactory fixing solutions are the gum arable (Flogel- 

 Schultze) and collodion-clove oil (Schallibaum) mixtures. 



The gum-arabic method is carried out as follows : of a saturated 

 aqueous solution of best gum arabic (a crystal of thymol being added 

 to prevent the growth of fungi) about 12 drops are added to 30 c.c. 

 of distilled water and thoroughly shaken. The slide is flooded with 

 the solution, care being taken that the fluid does not run over the 

 edges, and the sections are floated on the liquid, every part of the 

 sections being separated from the slide by the stratum of solution : 

 when all the sections are arranged, the slide is transferred to a warm- 

 ing-plate and very gently heated to a temperature never as high as 

 the melting-point of the paraffin, the object being to enable the 

 sections to expand while swimming on the gum solution ; this they 

 do in a most satisfactory manner within a few minutes, the sections 

 spreading out perfectly flat even when previously wrinkled. 



After expansion the excess of the fluid is drained off, and, if neces- 

 sary, the sections finally rearranged ; the slides are then placed in a 

 suitable place to dry, where evaporation is favored but protection 

 from dust is afforded. It usually is best to allow the sections to lie 

 overnight to insure complete drying, as if water be still present the 

 sections will not properly clear up. 



The disadvantages of the method are the long time required to 

 insure complete evaporation of the fluid and the inability of sections 

 so fastened to withstand watery solutions, which dissolve the gum 

 and loosen the sections. These objections, however, are more theo- 

 retical than real, and are more than compensated by the superior 

 preparations secured by this method ; in the exceptional cases where 

 it is necessary to apply aqueous solutions, advantage may be taken 



