The fine structure of developing and differentiating mucilarge gland of Undaria pinnatifida Suringar were observed. As the mucilarge glands develope rapidly in the early stages of development，the observations were carried out by electron microscope when the leaves were 0.5-10.0 cm in length. The materials were taken in Akashi city on December 1978. The resu1ts obtained were as follows. The early young leaf period was approximatelly 0.5 cm in length，and characterized by a monolayer of undifferentiated cells. The original cells of the mucilarge gland were seen in these cells. The mucilarge glands were developed by fusion between epidermal neighbor cells.
In the middle young leaf period of 3- 5cm，laminaria structures and mucilarge glands were developed from the epidermallayer. The late young leaf period was approximately 10 cm in length. The differentiation of the tissue was almost completed in the late young leaf period. Even in these stages newlly developing mucilarge gland were observed in the epidermal layer.