These are
nuclear remnants, left behind when the nucleus was extruded from the erythroblast.
In normal individuals they are removed ('pitted') by the spleen, so that
they are not seen in the blood. When the spleen has been removed, or is
not functioning properly (hyposplenism), Howell-JollyBodies appear in the
peripheral blood. In some conditions involving dyserythropoiesis, a sufficient
excess of Howell-Jolly Bodies may be formed, so that even a normally functional
spleen may be unable to remove them all, but this is rare.