WHILE it is now generally accepted that the cell plays a dominant part in the fibrogenesis in connective tissues, the mechanism of the formation of the fibrils of collagen and those of elastic tissue and cartilage is completely unknown. The object of this communication is to direct attention to the presence of sub-microscopical filaments in the cytoplasm of fibroblasts, osteoblasts, osteocytes and chondroblasts. The present observations were made with the electron microscope and using ultra-thin sections of tissue.