After cutting in gum, the sections are gently removed from the knife into distilled water by a soft camel’s hair brush, and left there for an hour or two, until the medium is entirely dissolved out. They may then be stained and mounted, or they may be put away in spirit for an indefinite time, and then stained and mounted. *Celloidin* is for many purposes almost an ideal embedding medium. (1) It has great penetrating power; (2) it can be made of an admirable consistence for cutting purposes; (3) after sections are made it allows them to be very freely manipulated without fear of injuring them: (4) and being perfectly transparent and homogeneous in thin sections, it does not require to be removed from a section before mounting. It is insoluble in water, and in weak spirit; slightly soluble in alcohol of more than 90 per cent. strength, and very readily soluble in ether, or in a mixture of alcohol and ether. The last solvent is the one commonly employed. The embedding solution is made thus:-- Take some pure celloidin (“Schering’s,” sold in boxes containing an ounce of shavings, is very good) and pour on it about eight times its volume of a mixture of equal parts of absolute alcohol and ether. Allow this to stand all night until the celloidin is dissolved. The solution should be made about the consistence of ordinary mucilage. It is also convenient to have a thinner solution made by using double the proportion of alcohol and ether. Both solutions should be kept in wide mouthed stoppered bottles, and the stopper should be well greased with vaseline as an additional obstacle to the evaporation of the volatile ether.