Together with the developing embryo sac, the nucellus also changes. Two types of developmental changes are recognized, resulting in either tenuinucellate or crassinucellate ovules. The majority of angiosperms possess tenuinucellate ovules in which most of the nucellus degenerates before the embryo sac reaches maturity, subsequently leaving the mature embryo sac in direct contact with the inner integument. In tenuinucellate ovules, the inner epidermis of the integument that borders the embryo sac frequently differentiates into a specific tissue, the endothelium (or integumentary tapetum). It consists of radially elongated cells rich in cytoplasmic content. In crassinucellate ovules, the nucellus expands by cell division during embryo sac development, and the mature embryo sac is surrounded by a massive nucellus.
