Erythroblast denucleation in the peripheral blood was studied by electron microscopy. Blood was used from dogs anemic either by infection with Babesia canis or from injections of phenylhydrazine hydrochloride. One of the earliest stages of denucleation was the migration of nuclei to the plasmalemma. Mitochondria and coalesced vesicles, derived from the cell membrane of the erythroblast, congregated at the underside of the nuclear envelope unapposed by erythroblastic cell membrane. The coalesced vesicles apparently provided the limiting membrane which surrounded the deep circumference of the extruded nucleus and its associated hemoglobin rim, and also furnished a new plasma membrane for the cell in the area where the nucleus, in denucleation, had utilized the original erythroblastic plasmalemma.

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