Various methods have been used in order to overcome the difficulties attendant upon the carrying of a minute organism, such as a protozoon, through the numerous reagents necessary for imbedding in paraffine preliminary to microtome sectioning. It has been found that the phenomenon of coagulation in blood plasma makes it of use in work of this type. In carrying out this method, a few drops of blood plasma is placed on a depression slide under a binocular microscope and the specimen at once placed in it with a pipet. After the plasma coagulates, which will take place in a very few minutes, the specimen will be found to be firmly imbedded in a resistant fibrin clot which can be taken through the various reagents, imbedded in paraffine, sectioned in any desired plane and finally mounted on a slide. In brief, the clot containing the specimen may be treated as a regular piece of tissue of the same size.The orientation of the specimen in the clot may be accomplished before the clot is fully formed or at the time...