When the circulation of the blood is observed in the transparent tissues of an animal, it is noticed that the coloured corpuscles run in the axial part of the stream, while the colourless mostly keep in the peripheral still current. The coloured corpuscles move much faster than the colourless; they have also a gliding, while the colourless have a rotatory, motion. Further, if the frog's web be examined in the upright position, with the microscope inclined so as to be horizontal with the table on which it is placed, it is noticed that the great majority of the leucocytes not only flow in the peripheral stream, but on the upper surface of the vessel. From a number of observations of blood-vessels in the frog's web, examined in this position, it was found that for every 13 leucocytes which are seen running along the lower surface, there is an average of 92 on the upper. In fact, the only time, apparently, in which a leucocyte gets to the lower surface, is in passing round a curved capillary, where, in changing its position, the stream of coloured corpuscles prevents it from gaining the upper surface of the vessel for some distance. If followed along the stream, sufficiently far, such a leucocyte is eventually found to make its way through the stream of coloured blood-corpuscles and to gain the upper surface of the vessel.