The plasma membrane in immature cells is often irregular in contour. Some irregularities become conspicuous folds that continue to enlarge into the cytoplasm. These invaginations may continue to increase in size and typically expand into the central vacuole. Sections show two closely parallel membranes in areas where the invagination projects into the vacuole. A narrow layer of cytoplasm may traverse the intermembrane zone between the membranes. The interior of an invagination may lack obvious content, or may be occupied by a fibrous material, or vesicular and tubular structures. The small vesicles bound by a single membrane appear to be derived from a projection formed most frequently near the orifice of the invagination. The origin of large vesicles and tubules possessing one membrane is not certain although they may arise by the fusion of several small vesicles. Alternatively, tubules, once formed, often possess constrictions along their length which suggest that these structures may become divided into a series of smaller vesicles. There is some evidence that cytoplasmic vesicles may fold into the membrane of the invagination and subsequently be pinched off into the interior of the sac. These vesicles are bounded by two membranes. Vesicles frequently contain an electron‐dense content more or less homogeneous in composition and unlike the typical ribosomal character of the cytoplasm. The function of the invaginations or their content remains to be elucidated.