Final answer:
The reticulocyte stage is where an erythrocyte ejects its nucleus during maturation, making room for hemoglobin critical for oxygen transport.
Step-by-step explanation:
The stage of development in the life of an erythrocyte during which the nucleus is ejected is known as the reticulocyte stage. As an erythrocyte matures in the red bone marrow, it extrudes its nucleus to make room for more hemoglobin, which is crucial for oxygen transport. Once released into the bloodstream, the reticulocytes mature into erythrocytes, which then circulate for an average of 120 days before being destroyed by macrophages.