The author had been enabled to study the blood corpuscles under circumstances capable of extending our information with regard to their relations, mode of formation, and ultimate destination. In 1845, he had discovered a peculiar condition in human blood, in which the colourless cells were greatly increased in number. This condition he had called leucocythemia, which was always associated with enlargement of the spleen or other lymphatic glands, a circumstance which had induced him to form the opinion that the corpuscles of the blood originated in these organs.