One of the nucellar cells situated subjacent to the epidermis differentiates into the primary archesporial cell. This cell is larger than the adjacent cells has dense cytoplasm and prominent nucleus. In tenuinucellate ovules, the archesporial cell functions directly as the megaspore mother cell (abbreviated as MgMC). In crassinucellate ovules, the archesporial cell first divides transversely to cut off a primary parietal cell on the outer side, and a primary sporogenous cell on the inner side. The primary parietal cell may either remain undivided or it may undergo a few divisions to form a few cells above the sporogenous cell.
Figure: A part of a young ovule with large megaspore mother cell