Printed in Great Britain THE ULTRASTRUCTURE OF THE CEMENT GLAND IN XENOPUS LAEVIS

Alterations which occur during differentiation in the fine structure of the cement gland of the embryo of Xenopus laevis have been investigated. The organ anlage at the late neurula stage is composed of cuboidal cells of comparatively simple cytoplasmic structure. Coincident with the subsequent cellular elongation there is a formation of extensive arrays of functionally inter-related membrane systems, leading to the production of a mucin-like secretory substance. Although there is no direct structural continuity between the membranes of the endoplasmic reticulum and the Golgi apparatus, small vesicles which seem to originate from areas of agranular reticulum appear to transport material synthesized in the endoplasmic reticulum to the Golgi zone. Further elaboration of the product at the site of the Golgi material is suggested by the progressive increase in the quantity of the contents as the cisternae enlarge to form secretory granules. Other notable features of the differentiating cells are microtubules and cytoplasmic filaments, many of which are oriented in the direction of cellular elongation. In suitably preserved specimens, large numbers of glycogen granules are present. The degenerating gland is charac-terized by the appearance of large autolytic vacuoles within the cytoplasm. Traces of the membrane systems are present and, in many cells, there remain large numbers of secretory granules