Pus cells are very like colourless corpuscles, and the relation between the two has been much debated. A pus cell can be distinguished from a colourless blood cell only by its mode of origin. If it have an origin external to the blood, it must be pus; if it originate in the blood, it must be considered to be a blood cell. In the early stages of its development, a white blood corpuscle is seen to modify by division; but in fully-developed blood such division is never seen. It is probable that colourless white corpuscles are given to the adult blood by the lymphatic glands. Every irritation of a part which is freely connected with lymphatic glands increases the number of colourless cells in the blood. Any excessive increase from this source I have designated _leucocytosis_. In the first months of the embryo the red cell multiplies by division. In adult life the mode of its multiplication is unknown. They, also, are probably formed in the lymphatic glands and spleen. In a disease I have named _leukaemia_, the colourless blood cells increase in number enormously. In such cases there is always disease of the spleen, and very often of the lymphatic glands. These facts can hardly, I think, be interpreted in any other manner than by supposing that the spleen and lymphatic glands are intimately concerned in the production of the formed elements of the blood. By _pyaemia_ is meant pus corpuscles in the blood. But most cases of so-called pyaemia are really cases in which there is an increase of white blood corpuscles, and it is doubtful whether such a condition as pus in the blood does ever occur. In the extremely rare cases, in which pus breaks through into the veins, purulent ingredients may, without doubt, be conveyed into the blood, but in such cases the introduction of pus occurs for the most part but once, and there is no persistent pyaemia. Even when clots in veins break down and form matter like pus, it will be found that the matter is not really pus, and contains no pus cells. _Chlorosis_ is a condition in which there is a diminution of the cellular elements of the blood, due probably to their deficient formation in the spleen and lymphatic glands. _The Vital Processes and Their Relation to Disease. Inflammation_