The wonderful migration of the leucocytes, which seems to show a conscious protective action on their part, takes place under the action of conditions which influence the movement of cells.  When an actively moving amoeba is observed it is seen that the motion is not the result of chance, for it is influenced by conditions external to the organism; certain substances are found to attract the amoebae towards them and other substances to repel them.  These influences or forces affecting the movements of organisms are known as tropisms, and play a large part in nature; the attraction of various organisms towards a source of light is known as heliotropism, and there are many other instances of such attraction.  The leucocytes as free moving cells also come under the influence of such tropisms.  When a small capillary tube having one end sealed is partially filled with the bacteria which produce abscess and placed beneath the skin it quickly becomes filled with leucocytes, these being attracted by the bacteria it contains.  Dead cells exert a similar attraction for the large phagocytes.  Such attraction is called chemotropism and is supposed to be due in the cases mentioned, to the action of chemical substances such as are given off by the bacteria or the dead cells.  The direction of motion is due to stimulation of that part of the body of the leucocyte which is towards the source of the stimulus.  The presence in the injured part of bacteria or of injured and dead cells exerts an attraction for the leucocytes within the vessels causing their migration.  When the centre of the cornea is injured, this tissue having no vessels, all the vascular phenomena take place in the white part of the eye immediately around the cornea, this becoming red and congested.  The migration of leucocytes from the vessels takes place chiefly on the side towards the cornea, and the migrated cells make their way along the devious tracts of the communicating lymph spaces to the area of injury.  The objection may be raised that it is difficult to think of a chemical substance produced in an injured area no larger than a millimeter, diffusing through the cornea and reaching the vessels outside this in such quantity and concentration as to affect their contents, nor has there been any evidence presented that definite chemical substances are produced in injured tissues; but there is no difficulty in view of the possibilities.  It is not necessary to assume that an actual substance so diffuses itself, but the influence exerted may be thought of as a force, possibly some form of molecular motion, which is set in action at the area of injury and extends from this.  No actual substance passes along a nerve when it conveys an impulse.