ABSTRACT
The bone marrow cavity develops in the humerus, tibia and femur of the chick embryo on the eighth or ninth day of incubation. An initial perforation into the region of the future medullary cavity in the developing bone is made by the action of osteoclasts breaking down a locus in the external osseus wall of the otherwise cartilaginous model of the bone. The immigration of hemopoietic cells into the chick bone marrow promptly follows the establishment of the marrow parenchyma. Once blood cell production is inaugurated in the embryonic bone marrow erythrocytes as well as granulocytes and thrombocytes are concurrently generated. Erythrocytes are probably produced in greater number than granulocytes. Candidate stem cells believed to be committed to the erythroid lineage and basophilic erythroblasts are seen to establish extensive contacts with and adhere to the lumenal surface of the formative venous sinuses.
