The detailed vascular structure of the bone marrow of saponin-treated mice was studied with clarified bone specimens (Spalteholz's method), because the technic appeared encouraging in elucidating the problems of the mechanism of both granulocytic liberation into the peripheral circulation and development of the extramedullary hematopoiesis. The results obtained were as follows. 1. In the normal control, unit sinuses were connected with each other, and a unit sinus net was primarily composed of 6 unit sinuses in a hexagon form, and many of these sinus nets were mutually connected to make up the complex network. The sinus wall was composed of a single layer of endothelial cells. The vascular system of the bone marrow was a completely closed system, no fenestrated vessel-walls being demonstrable in normal mice. 2. In saponin-treated mice the sinus network was disordered. There was enlargement or narrowing of the sinuses with destruction of their wall. These alterations were seen already at 24 hours after an injection of saponin to mice. At this stage, no extramedullary hematopoiesis was yet seen in spleen tissue culture or in histologic specimens of the spleen. It was considered, therefore, that saponin acted upon sinuses causing destruction of their thin walls. This was followed by the liberation of blood cells into the peripheral cirulation from the destructed sinuses. Finally, these blood cells were transported and multiplied in the organs such as the spleen and liver. The author believe that this is the mechanism of the extramedullary hematopoiesis (colonization theory). The simultaneous finding that megakaryocytes increased in number in the spleen, while their number decreased in the bone marrow gave support to this view.