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The fiber cells of the middle layer of Trichoplax adhaerens are interconnected by slender extensions. Newly formed connections after mechanical disruption of the tissue studied in ultrathin sections revealed cytoplasmic continuity between the cell bodies, suggesting a syncytial organisation of the fiber cell layer. The slender extensions connecting the cell bodies are traversed by microtubules and microfilaments. The structure of rare osmiophilic cell contacts suggests a stage in the fusion of adjacent cell membranes.

Cambarus virilis. The staining of thick sections, cut alternately with thin sections for electron microscopy, has permitted identification of the basophilic bodies with two types of lamellar systems. One of these, a set of straight annulate lamellae, is restricted to meiotic prophase. The second type of lamellar systems has been found from late prophase to early spermatid stages. It consists of an ellipsoidal lamellar set which intersects a number of straight lamellae. Within the region of intersection, the ellipsoidal lamellae break up into an array of small tubules of about 150 A diameter. The term tubulale lame.liar system was chosen to designate this type of lamellar complex. Small RNA-coutaining granules could not be detected in annulate lamellar systems. While there are a few granules in the marginal regions of the tubulate lamellar system, their distribution cannot be responsible for the basophilia which is intense within all regions of the lamellar body. INTRODUCTIONStudies on cellular fine structure as revealed by the electron microscope have shown that the ground substance of the cytoplasm of many cell types is extensively partitioned and contains numerous membrane-bound cavities. The portion of the whole cytoplasmic volume occupied by such cavities or cistemae, their arrangement and degree of branching, is variable and frequently characteristic of the cell type. This whole spectrum of small vesiculate elements, limited by what appears to be a single membrane at the present levels of resolution, and characterized by a homogeneous content, has been subsumed under the name endoplasmic reticulum (19). Morphologically, two varieties of endoplasmic reticulum are recognized today, depending upon the presence * This paper is respectfully dedicated to Dr. W. E. Ankel, Professor of Zoology, Giessen, Germany, on occasion of his sixtieth birthday.Much of the work reported here was made possible by a National Science Foundation Grant to j . o . Gan.§ Present address: Whitman Laboratory, The University of Chicago.or absence of small granules of about 150 A diameter on or near the outside of the vesicles. Granular endoplasmic reticulum, first described from exocrine cells of the pancreas (16,21), is found to be extensively developed in cells engaged in protein synthesis. There is a substantial body of evidence which points to the small granular component as the possible site of cytoplasmic ribonudeic acid (RNA). Palade and Siekevitz have recently isolated these small particles from guinea pig pancreas by differential centrifugation (18). They found the postmicrosomal fractions containing the granules to have a higher RNA/protein ratio than samples containing both granules and small vesicles of the endoplasmic reticulum. Clermont (7) found the cytoplasmic RNA in the rat spermatid confined to dense granular aggregates. Both results indicate that at least some of the cytoplasmic RNA is localized within the small granules. There are, on the other hand, indications that a membrane-bound RNA fraction may exist in the cell...