In freshly isolated cells of the guinea pig germinal epithelium examined with phase contrast, dark contours are seen in the cytoplasm that appear to be optical sections of the cisternae of the endoplasmic reticulum. These increase in contrast, in number, and in linear extent with increasing time up to 4 hours after isolation of the cells from the testis. During this period, cisternae originally present in the cells are extended and new ones appear to be formed by coalescence of tubular and vesicular elements of the reticulum. The cisternae become associated in parallel array and ultimately form elaborate concentric systems resembling structures that have often been interpreted as intracellular "myelin figures." Until now our knowledge of the endoplasmic reticulum has been based largely upon electron micrographs. The observation that the cisternae are visible in certain cell types under phase contrast optics opens the way for experimental investigations on the behavior of this class of cytoplasmic membranes in living cells.

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