1. =CELLOIDIN IMBEDDING.= The granular form of Schering’s celloidin is the best preparation to use, although good results can be obtained by using a cheaper well-washed gun-cotton. In purchasing the latter care should be taken to secure a sample that dissolves easily in alcohol and ether, and does not give off yellow fumes when exposed to the light. Schering’s granular celloidin keeps well, and forms on solution a firm, tough, transparent imbedding mass, so that thin sections are obtainable without difficulty. When kept long in stock celloidin becomes hard and dissolves more slowly. For use three solutions are made, thick (10 per cent), thin (2 per cent), and medium (5 per cent). The celloidin granules or shavings are put into a wide-mouthed bottle having a tight stopper, and are covered with absolute alcohol and well shaken, and left for 24 hours. An equal quantity of pure ether is then added, the mixture is well stirred and allowed to stand for another 24 hours, when it is again stirred and evenly mixed, and is then ready for use. When gun-cotton is used it is torn into fine shreds and added to a mixture of equal parts of absolute alcohol and pure ether and shaken until sufficient has been added to give the solution the desired strength. Slow Celloidin Method. 1. Absolute alcohol 24 hours. 2. Equal parts absolute alcohol and ether 24 hours. 3. Thin celloidin for 1-3 days. 4. Medium celloidin for 1-3 days. 5. Thick celloidin for 1-3 days. 6. Block. The tissue is blocked by removing it from the thick celloidin on a section-lifter and placing it on a block of vulcanized fiber or wood with enough of the thick celloidin about it to form a good matrix. The preparation is then allowed to evaporate in the air until the surface of the celloidin becomes firm (does not stick to the finger). The block is then placed in 80 per cent alcohol or pure chloroform until hard enough for cutting (1-24 hours). Cork should not be used for blocking, nor should wood unless the tannin has been removed by long treatment with alcohol-ether. The celloidin will adhere more firmly to the fiber block if the latter is dry, and if there is a sufficient layer of celloidin between the tissue and the block. The imbedded tissues may be preserved in 80 per cent alcohol for a long time, but gradually lose their staining power. They may also be kept dry by coating them with melted paraffin. The blocks when preserved in alcohol may be marked with an indelible pencil.