The advent of red blood cells where grossly the exudate is red can hardly be explained by the simple process of diapedesis. Indeed, there is ample evidence that they escape by rhexis through lesions of the vascular wall. This phenomenon cannot be demonstrated in areas where a compact mass of red cells obliterates the alveolar space (Fig. XXV), but in the less firmly consolidated alveoli where red blood cells predominate (Fig. XXVI) the picture of the alveolar wall is very instructive. The capillaries may be prominent and contain red cells almost exclusively. Often the epithelium of the air space is exfoliated so that there is nothing to minimize the prominence of the engorged vessels. These capillaries, covered by such a delicate wall that rupture seems imminent, may protrude like saccular aneurysms (101) into the alveolar space (Fig. XXVI). In all probability, these sacs do rupture and this result would be one explanation for the escape of large numbers of red blood cells. In several instances such a picture was encountered, where with little reaction at the point of rupture, red cells within the vessel were continuous with an accumulation of similar cells in the alveolus. Further evidence for such rupture is offered where the vessel is collapsed. Here there is accumulation of polymorphonuclear leucocytes in the area of destruction in contrast to the well preserved red corpuscles in the remainder of the vessel (Fig. XXVI). The above pictures may be utilized in the interpretation of the outspoken foci of red hepatization which may assume infarct-like proportions. As has been said, it is impossible to conceive that these hemorrhagic areas where the alveoli are packed with red cells ever change to a grey type of consolidation. Consequently, it seems more probable that the color of red hepatization in the usual types of pneumonia depends upon the marked engorgement of the vessels seen through a relatively acellular, transparent, serofibrinous, alveolar mass and not upon the number of red cells in the exudate. C. THE APLASTIC EXUDATE