The Red Corpuscles are disc-shaped bodies whose chief constituent is haemoglobin. The origin of these corpuscles cannot yet be said to have been unequivocally determined. In the early embryo they are formed coincidently with the blood-vessels, and the cells that develop into the latter give origin to the red corpuscles, which are thus intracellular products. This method of production, however, ceases before birth, and in the embryo itself, and in the individual throughout extra-uterine life, a different mode of origin must be looked for.

The bone-marrow is agreed upon by many authors as the chief permanent seat of origin of the red corpuscles. In the red marrow of the cancellous tissue of bones, and more particularly of the ribs, the venous sinuses present special nucleated cells (erythroblasts) which give origin to red corpuscles. Whilst the bone marrow is the chief source, the spleen and other structures also probably take part.

A great destruction of red corpuscles is continuously taking place, as the bilirubin in the bile is entirely derived from the haemoglobin of the corpuscles. The destruction of the corpuscles probably occurs in the general circulation as the termination of their cycle of life, and no special destroying agent or seat may be necessary. This great and constant loss of corpuscles implies an equal new-formation and replenishment.