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It is generally agreed that the inner of the two membranes that enclose the photosynthetic compartment of chloroplasts evolved from the plasma membrane of a prokaryotic symbiont. The source of the outer of the two membranes is perhaps less certain. The earlier view was that it had evolved from the (endo)membrane of the vacuole in which the symbiont had been sequestered following endocytosis by a eukaryotic host cell. This hypothesis has been challenged on the bases that, firstly, the outer envelope membrane differs in its chemical composition from eukaryotic endomembranes but resembles that of prokaryotic membranes and, secondly, in spite of repeated claims, there is no substantial evidence that the outer membrane ever becomes continuous with other components of the host cell’s endomembrane system.