liver and spleen, of certain very peculiar cells. These cells have long been known as giant cells, and all attempts at accounting for them have broken down. They are relatively of immense size: their diameter may be twenty times as great as that of a red blood-corpuscle. Each contains a huge irregular, bulging nucleus. Hence the cells are termed “megacaryocytes” (big-nucleus cells). They must not be confounded with the polycaryocytes (cells with several nuclei), which eat up degrading bone, although it must be confessed that megacaryocytes and polycaryocytes appear to be genetically connected. It is supposed that megacaryocytes consume the nuclei which red corpuscles extrude during the process of their conversion from nucleated cells. Traces of nuclei, or things which often look like nuclei, are found in their body-substance. Their own overgrown misformed nuclei appear to be the result of an excess of nuclear food. It is certainly remarkable that megacaryocytes are not found below mammals. They do not occur in any animal in which red blood-corpuscles retain their nuclei. Polycaryocytes are found in numbers in the bones of growing birds. They are evidently scooping out bone from situations in which it has to be displaced in order that the shape of the bone as a whole may be changed. But there are no megacaryocytes in birds. On the other hand, megacaryocytes are present in the liver, and later in the spleen, of mammals at the periods when blood-formation is occurring most actively in these organs. From the liver they disappear early. In most mammals they disappear from the spleen about the time of birth; but in some—the hedgehog, for example—they are found in the spleen throughout the whole of life. Hæmoglobin is a substance which has the property of uniting with oxygen to form oxyhæmoglobin—a compound from which the oxygen is, again, very readily withdrawn. It is extremely soluble, but may be made to crystallize by adding alcohol to blood, after setting the hæmoglobin free from the corpuscles by freezing and thawing. From the blood of Man and most other animals it crystallizes in the form of rhombic prisms, whether in the oxidized (oxyhæmoglobin) or non-oxidized condition. The addition of oxygen does not affect its crystalline form; although crystalline, it is absolutely non-diffusible. This is due to the great size of its molecule, which is probably larger than that of any other substance which is capable of crystallizing. The percentage composition of hæmoglobin conforms closely with that of albumin and other proteins, with this most important difference: it contains a definite proportion of iron—0·336 per cent. That the percentage of carbon, hydrogen, nitrogen, sulphur, and oxygen should agree with that commonly found in proteins is inevitable, since it may be split into a part which contains all the iron, hæmatin, and a proteid part resembling albumin; and the latter constitutes 96 per cent. of its weight.