A combined histological and electron microscopic study of the process of leucocytic emigration in rat skin induced by intradermal injection of homologous serum has been made. By the intravenous injection of colloidal suspensions of carbon black and mercuric sulphide it has been shown that leucocytes may pass through the vessel wall with no concurrent escape of injected particles, and presumptively of plasma proteins. That is, total dissociation between leucocytic emigration and vascular permeability has been demonstrated.

Similar studies following histamine injection have confirmed previous reports that “gaps” form between adjacent endothelial cells in vessels in a state of increased permeability. These “gaps” do not allow leucocytes to emigrate in the absence of the appropriate specific stimulus.

The significance of the findings on the function of the several layers of the wall of small blood vessels is discussed.