According to the concept detailed by Jordan and which continues to enjoy mainstream support, the small lymphocyte enters the marrow venous sinuses usually by migratory perforation and acts as the erythropoietic stem cell. Intrasinusoidal erythropoiesis in the bone marrow of regenerating marrow in the post-starved, re-fed pigeon has been analyzed on the basis of sections and imprint preparations by McDonald. Examination of marrow intrasinusoidal erythropoiesis under electron microscopy yields information that supports and extends the interpretations of erythropoiesis derived from light microscopy. The transversely positioned cell whose margin touches the right border of the frame immediately below an imaginary line dividing the plate into upper and lower halves is an endothelial cell lining a bone marrow sinus. An electron photomicrograph of the femoral hemopoietic bone marrow of Alectoris rufa rufa the red-legged partridge. Basophilic erythroblasts have a decreased size and number of mitochondria, less rough endoplasmic reticulum and a reduced Golgi component in comparison with the hemocytoblast.