=Gelatin method= of mounting Kaiserling preparations: Soak washed Gold Label gelatin in distilled water for 12-24 hours. Take equal parts of water-logged gelatin and glycerin and dissolve by heating in a double boiler, stirring, for 15-20 minutes. Cool to 40°C. Then clarify with white of egg (well-whipped whites of three eggs to half a gallon of jelly: stir well; steam for ½ hour) and filter through cotton-wool. Add to jelly a few drops of a weak aqueous solution of crystal violet to remove yellow color (_Bruère and Kaufmann_). To prevent growth of bacteria a small percentage of formol or crystal of phenol may be added. Kaiserling specimens are placed in glass dishes in the melted jelly, and covered with it. When set the dishes containing the mounted specimens may be covered with glass-plates and fastened to these by balsam or cement. I use a deep Petri dish, filling it about two-thirds full with the jelly; over this I pour melted paraffin of a very low melting point so as not to melt the jelly. When the layer of soft paraffin is hard, a thick layer of paraffin of a 52° melting-point is poured over it, and the dish filled even. When the hard paraffin is set, it is varnished with shellac. Liquefaction of the gelatin by bacteria or enzymes constitutes the great drawback to this method. CHAPTER XX. THE FIXATION AND HARDENING OF TISSUES. =GENERAL CONSIDERATIONS.= For the examination of material that may be injured by freezing, or when very thin sections are required for complicated staining procedures, it becomes necessary to prepare the tissue by _fixation_ and _hardening_, so that it can be _imbedded_ in some medium permitting the _cutting_ on the microtome of as thin sections as may be desired. Fixation is that process by which the appearances of the tissue are preserved as they were when it was taken for examination; hence in order to obtain pictures resembling as closely as possible those of the living tissue the material should be _fixed_ immediately upon its removal from the living body by operation, or as soon as possible after death when obtained by autopsy. _Fixing agents_ act by coagulating the cell albumins, in this way “setting” or “fixing” the constituents of the cell so that further change is stopped. Fixation, therefore, _hardens_ the cell, and all fixatives are also hardening agents. A practical distinction between fixing and hardening is made, however, resting upon the fact that not all fixatives harden the tissue so completely that the proper consistence for the cutting of thin sections is attained. To achieve this the tissue must be dehydrated. Alcohol and acetone are the only reagents fixing and hardening perfectly at the same time, as they remove the water from the tissue; for all other fixing agents an _after-hardening_ in alcohol is necessary. In the case of such reagents the division of the process into a _primary fixation_ and