Cells from the anterior segment of the salivary glands of Sciara coprophila were found to synthesize and secrete into the gland lumen three morphologically distinct types of granule: 1) A large, electron-lucid granule, up to 1 µ in diameter, staining only faintly with pH 2 fast green and the PAS reaction; 2) an ellipsoid granule of moderate density, strongly fast green and PAS positive; and 3) a small spherical granule of high electron density. The cells contained numerous Golgi areas, up to an estimated 8,000 per cell. Evidence is presented for the transfer of material from the endoplasmic reticulum to the Golgi areas via small vesicles. Three types of Golgi areas were distinguishable, each containing intercisternal material resembling one of the three types of secretion granule. Patterns of secretion granule synthesis varied with the developmental stage of the larva as determined by counts of eye spots in the eye anlage. Lucid granules were most abundant in the youngest larvae, and decreased in abundance as larvae grew older, becoming virtually absent in prepupae. The small, dense granules were present in all gland cells, but became more prevalent in older larvae and prepupae. Ellipsoid granules were only occasionally present, and were independent of larval stage. It is suggested that lucid granules are digestive in function, since their abundance correlates with feeding patterns. Other granules may produce the external slime coating of the larvae, and also the mucoprotein component of the pupal cocoon.

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