The formation of red blood cells (erythrocytes), which occurs in the red bone marrow (see haemopoietic tissue). The earliest precursor that can be distinguished microscopically is the proerythroblast, which develops from a haemopoietic stem cell and gives rise successively to the early erythroblast, intermediate erythroblast, and the late erythroblast, in which most haemoglobin is synthesized. In mammals the nucleus is then forced from the cell, which assumes a biconcave shape and is known as a reticulocyte. Reticulocytes are released into the blood and develop into mature erythrocytes within two days. See also erythropoietin.