Two successive stages — nucellar and archesporial — of ovule development in three hybrids ofDendrobiumorchids have been studied by electron microscopy. A megaspore mother cell is developed at the tip of each placental outgrowth following a 30-day mitotic period induced by pollination. Except for its terminal position in the nucellar row, the potential megaspore mother cell exhibits no unique morphological properties. It is proposed that the marked multiplication of organelles effecting cytoplasmic growth in the archesporial phase is facilitated through a net absorption of cytoplasm from the surrounding nutritive cells by the developing megaspore mother cell. Evidence for this is presented by the concomitant loss of plasmodesmata and the initiation of rhopheocytosis between the developing cell and the adjacent nucellar and epidermal cells and a marked reduction in the concentration of ribonucleoprotein particles within the developing cell. No evidence can be found that the simple Golgi-apparati of the archesporial cell elaborate either the numerous single-membrane-bound vesicles of the cytoplasm or the extensive, continuous network of the cell endoplasm. Furthermore, no evidence can be found to support the theory that mitochondria and plastids share a common origin or that their origins arede novo.Cup-proliferation and binary fission are the methods by which they are duplicated in the developing archesporial cell. Cytoplasmic lipid microdroplets are also described and discussed.