Before embedding a specimen it is necessary to dehydrate it thoroughly for twelve to twenty-four hours in absolute alcohol. It should then be placed in a mixture containing alcohol and ether for an hour or two, and afterwards transferred to the thin solution of celloidin for twenty-four hours, and then to the thick solution for the same period. The celloidin penetrates slowly and in the case of nerve tissues and other delicate structures it is wise to give the full allowance of time for the different steps. When the tissue has been thoroughly permeated by the celloidin, it is gently removed from the celloidin and placed in position on a piece of cork of suitable size for clamping in the holder of the microtome. Celloidin is painted round the object so that it is supported on every side. It is then left exposed to the air until the surface has become firm, when the cork is placed, with the tissue downwards, in methylated spirit. The cork floats but the tissue and celloidin remain submerged. At the end of twenty-four hours the celloidin will have become semi-opaque and opalescent, and of the same consistence as hard boiled white of egg. When it is impossible to wait so long, rapid hardening of the celloidin may be secured by immersing it in methylated chloroform in place of spirit, but the slower method gives more uniformly satisfactory results. Pieces of tissue embedded in celloidin may also be cut on a freezing microtome. After the celloidin has become firm by immersion in methylated spirit, the tissue with the celloidin round it may be cut off the cork, washed in water to remove the alcohol, and then soaked for an hour or two in gum, placed on the plate of an ether spray microtome, frozen and cut in the usual way. Subsequent staining operations are conducted in the same way as for sections cut by hand or in gum. As celloidin is only slightly stained by hæmatoxylin, alum carmine, borax carmine, &c., it is not necessary to remove it from the sections, but it exhibits so intense a staining reaction with aniline dyes that it is necessary to remove it by treatment with alcohol and ether either before or after the staining operation. The sections after staining may be mounted in Farrant’s solution (p. 59), or in Canada balsam (p. 61). If the latter medium is employed, the section should be clarified, after dehydration in alcohol, by means of oil of bergamot, or oil of origanum, instead of oil of cloves, as the latter dissolves out the celloidin and causes the section to break up. Celloidin is most useful for cutting sections of the coats of the eye, of the internal ear, and of bone marrow. It should always be used for the Weigert-Pal hæmatoxyline method of staining the nervous centres, as it protects the section from being injured by the transference from one fluid to another which is repeatedly required during the process. The stain is completely discharged from the celloidin by the decolourising solution used (p. 90).