A histological and histochemical study of the haematopoietic nodule in the liver of Pastor roseus was made from October to April. The formation and development of the haematopoietic nodule by aggregation of lymphocytes and monocytes was found to be a continuous process, in which new nodules are formed regularly. The older nodules after a certain period of high activity show a considerable decline in the activity of blood-cell formation. The fully formed haematopoietic nodules give rise to different types of blood-cells--erythrocytes, granular leucocytes, lymphocytes, and monocytes. During the pre-migratory period the lymphocytes and monocytes were found to be more numerous in the nodules. It is likely that the number of erythrocytes increases during migration and that the formation of different types of blood-cells depends upon demand. It is concluded that fat is synthesized in the lymphocytes, monocytes, and free-moving reticulo-endothelial cells, but that this synthesis is mainly carried by the reticulo-endothelial cells. The phagocytes may also be the carriers of fat, and it is suggested that they are later modified into fat-cells in the fat-depot. Haematopoiesis in this bird is a normal phenomenon occurring in the liver in all seasons and is not a pathological condition.

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