Combinations of celloidin and paraffin may be employed by imbedding first in celloidin, transferring block to origanum oil, then xylol and finally paraffin. Formol-agar has been recommended by _Bolton_ and _Harris_ for simultaneous fixation and imbedding. It offers no especial advantages. _Wright_ uses formol-gelatin for imbedding tissues for sectioning on the freezing microtome. The bits of tissue are placed in warm 20 per cent pure gelatin; this is allowed to set, and the block placed in 10 per cent formol for 24 hours; it is then frozen and cut. Soap-gelatin, glycerin-gelatin, gum-glycerin, etc., are now rarely used for imbedding. NOTE:—When a number of pieces of tissue are blocked at the same time, either in celloidin or paraffin, they may be tagged with paper labels fastened to the tissue with a drop of gum tragacanth or gum arabic. As the gum is not soluble in any of the infiltrating and imbedding media the labels remain attached until the block is ready for cutting. CHAPTER XXIII. SECTION-CUTTING. Fixed and hardened tissues may be sectioned on a microtome without freezing or imbedding, or they may be cut on a freezing-microtome; or, having been imbedded in either celloidin or paraffin, they may be sectioned on any form of microtome suited to the purpose desired. The choice of the microtome depends, therefore, upon the character of the work. For ordinary purposes the small sliding microtomes furnished with a crank are preferable, as they are more easily kept in order and can be used for rapid cutting. For serial paraffin sections the Minot automatic rotary microtome is advised. Especial types of microtomes can be obtained for the cutting of large sections, particularly for brain-sections. The Bardeen freezing-microtome I have already recommended as the most practical instrument for freezing work. In all cases it is absolutely necessary that the instruments be in good order and that they work with precision. The microtome knife must be carefully honed and stropped. A heavy, rigid, biconcave knife-blade should be used, and a good hone and strop are absolute necessities. In honing, the knife is drawn from heel to toe with cutting edge forward; while, in stropping, the motion is reversed, the back of the knife being forward, and the motion from toe to heel. The knife during the honing should be fitted in a knife-holder, and the back of the blade protected from the hone by a knife-back. The knife-blade must be kept free from grease and dirt, and nothing must be permitted to touch its edge. When many sections are cut at one sitting frequent stropping is necessary. When the cutting is finished the knife should be removed from the microtome, carefully cleaned and dried, and placed in its box. The slide and other bearings of the microtome should be well-oiled with the best microtome oil, and kept free from dust.