The difficulty in the way of establishing a complete theory of the coagulation of blood increases when the phenomena of incoagulability are considered. Blood may be rendered incapable of clotting in a variety of ways. Leeches and other animals which suck blood have the capacity of rendering it incoagulable. If the heads are removed from a score of leeches, thrown into absolute alcohol, dried, ground in a pepper mill, extracted with normal saline solution, a dark turbid liquor is obtained. This liquor, after filtration and sterilization at a temperature of 120° C., injected into the veins of an animal, renders its blood incoagulable. The preparation sold by druggists under the name “peptone,” when injected into the veins of a dog, renders its blood incoagulable. Commercial “peptone” is a mixture of many substances. Its anticoagulation-effect is not due to the peptone which it contains. It has been supposed to be due to imperfectly digested albumin and gelatin (proteoses), but products of bacteric fermentation (toxins and ptomaines) are more probably the active bodies. Not only is the peptonized blood of a dog incoagulable, but if this blood be injected into the veins of a rabbit (an animal upon which the direct injection of peptone has no effect), it diminishes the coagulability of the rabbit’s blood. If peptonized blood be mixed in a beaker with non-peptonized blood, it prevents the coagulation of the latter. There is little doubt but that the poison, whatever it may be, acts upon the leucocytes; and there are some reasons for thinking that the poison is not contained in the “peptone,” but is secreted by the liver of the animal into which the “peptone” has been injected.