Aneurysms are not so common in birds, the incidence being in comparison with mammals as 1 to 2.2. Two seats are prominent for their development: the sinuses above the aortic valves and the first part of the subclavian vessels; two of the six cases occupied the first position, two the second, while one other lay in the arch of the aorta, the last in its descending thoracic portion. Those developing over the valves seem to arise from simple degenerative arteritis; those that occupy the other locations are apt to be surrounded by plaques on the intima. Aneurysms in birds reveal by microscopy some trace of all the vascular coats and seem not to construct an adventitia from surrounding areolar tissue. The veins have presented no peculiar pathology, except in tumors which will be taken up later. A Derby’s Tyran showed a phlebitis and periphlebitis of the left subclavian vein, of mycotic nature, which led to death by rupture and hemorrhage. SECTION IV DISEASES OF THE BLOOD AND BONE MARROW The production and physiology of the circulating blood seem closely similar in the two classes under consideration, although the anatomy is not the same in birds and mammals, variations also occurring within the latter group. Pathological responses follow comparable lines in that hemolyzing agencies, be they hemosporidia, absorptions from metazoan parasites or bacterial toxins, produce a reaction in erythropoietic centres, and positively chemotactic viruses call forth increases in the colorless elements. We have also observed a decrease of leucocytes in an Orang Utan suffering from influenza, a finding analogous to that in the human attack. There is, however, a much less ready response on the part of birds to any leucocyte-stimulating influence, in this class the mononuclears seeming to bear much of the burden assumed by the myeloid cells of Mammalia or at least appearing on the stage very quickly so that any increase of the latter is overshadowed by them. Perhaps this apparent difference may be further explained by the greater number of colorless blood cells, structures which might be called the principal secondary defences of the body and constantly at the disposal of the organism, normally present in the birds’ blood; they amount to 25,000 per cubic millimetre in birds, while in the mammals very few varieties have half this number. On the accompanying Table (6) will be found a few differential leucocyte counts now known to us.