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The structure of Euglena granulata and Chlamydomonas eugametos has been studied using polarization and electron microscopy, cinematography, and chemical extraction procedures, with the main focus on the structure of the eyespot.The 50-60 granules which form the extrachloroplastic eyespot of E. granulata are large bodies, up to 1200 mμ in diameter. They are found in the cytoplasm near the base of the reservoir and are associated with the parabasal body which contains a large crystal. The eyespot granules are contained within membranes having a unit membrane structure; 2 or 3 are usually present in a single eyespot packet; microtubules are also contained within the packet. The eyespot granules have the structure of a positive spherite and clearly exhibit birefringence; this structure is modified by fixation.The granules of the chloroplastic eyespot of C. eugametos are about 75 mμ in diameter and are contained within the chloroplast in an ordered array. Occasionally, the eyespot contains elongate or helical bodies mixed with the granules. Extraction with organic solvents caused the removal of materials which formed the eyespot granules as well as that of the osmophilic globules in the chloroplasts.Several hypotheses which concern the function of the eyespots in these and other species are discussed in the light of our results. the possible origin and demise of the eyespot granules are also discussed.

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