By preparing transparent tissue specimens by Spalteholz and stained tissue specimens with dye injection into the bone marrow vessels of thoracic vertebra in 60 human fetuses over two months old, the author studied development of blood vessels in the bone marrow and obtained the following resutls: 1. Ossification in the body of a vertebra commences with entochondrostosis, while that of the arch of a vertebra with exochondrostosis. 2. A pair of large blood vessels entering the cartilage canal from the foramen vertebrae grow into nutrient artery and vein. After entering into the bone marrow the nutrient artery branches off and anastomoses with each other; and in the latter half of the embryonal stage main branches, presenting ring formation, radially send out small branches and at the periphery, anastomosing actively with one another, they form networks. Veins, forming networks of venous sinus and having large vascular space, present a picture like intra-tissue spaces. 3. The nutrient artery and vein at the arch of vertebra enter from incisura vertebrae caudalis or from the underside of the base of spinal process and, immediately branching out, they run in the direction of each process and to the anterior part of the arch of vertebra. 4. The cartilage canal of the body of vertebra plays an important role in growth of the nutrient artery and vein, but those that do not penetrate into the bone marrow, though they branch out and develop, end in blind terminal. These gradually deteriorate after the peak of growth reached around the eighth embryonal month. The cartilage canals at the arch of vertebra are slow in their growth and a few in number. 5. In comparing blood vessels in the bone marrow of thoracic vertebra with those of long bone, it seems that the arch of a vertebra corresponds to the body of bone in the latter, and the body and the process of a vertebra correspond to the epiphysis in the latter.