The gall-bladder is not a constant organ in either mammals or birds and indeed it may be absent or present in very closely related species (Two- toed Sloth present, Three-toed Sloth absent). When present in mammals it is usually a dependent bag while in birds it commonly lies upon the cystohepatic duct, between the liver and the last curve of the duodenum, in some varieties filling from the bottom, the inlet being guarded by a valve. This cystic duct in nearly all birds, comes exclusively from the right lobe while the hepatic duct, with which the cystic has no connection, is formed by combination within the liver of radicles from both sides. It passes to the duodenum well in advance of the cystic duct, in some birds, _e.g._, the Struthiones, very near the pylorus, that is on the descending limb of the duodenal loop. By this means obstruction to the biliary stream is rendered difficult. The common duct combines with one of the pancreatic outlets in most mammals but the abdominal salivary gland in lower animals has more often patent separate ducts or multiple ducts than it does in man. Birds have one to four pancreatic ducts separate from the biliary openings. The gall-bladder is missing in most varieties of the following groups: pigeons, parrots, wrens, ostriches, rheas, cuckoos, toucans among the birds; most odd-toed ungulates, hyraces, Indian elephants, all deer, peccaries, three-toed sloth, and many rodents. The varieties lacking this reservoir are herbivorous in the main, true carnivores seeming always to be possessed of such a structure. Among the important herbivorous ungulates, Bovidæ, Tragulidæ, Camelidæ and Suidæ have this bile reservoir almost without exception. Because of the interest now being shown in the pathology of the gall-bladder and its passages and of the pancreas, it was hoped that evidence of definite practical value for human pathology would be at hand in our study if we divided the animals into groups with and without a bile reservoir. The result is not unequivocal but worthy of note; it is discussed on pages 238 and 255. Microscopically the well known lobular arrangement of the liver is rather faithfully carried out among the mammals albeit the most systematic and complete architecture is to be found in the pig while the marsupial seems the most disorderly, thus resembling the avian organ. In the latter class all the parts are indistinct, the cells having an unclear outline, the tubules being intricately wound and the interlobular connective tissue being scanty and not anastomosing in a definite framework. The intralobular reticulum is especially difficult to detect. Groups of cells are often found at portal spaces; these are large and small mononuclears and granular cells, probably of the hematopoietic system. It is possible that blood formation is performed in the liver and spleen in some adult birds but such a function is denied for the mammal except under very unusual conditions of bone marrow atrophy.