The fibroblasts of the pupa are characterized by the great development of the endoplasmic reticulum, which becomes dilated to form vesicles containing a rather electron-dense material which is thought to be a precursor of the collagen fibrils. Fibrils are seen within the cytoplasm of the fibroblasts; these are about 12.5 to 20 mµ. in diameter and some ofthem show indications of banding with a periodicity between 1 5 and 20 mµ. It is thought that these fibrils and their surrounding cytoplasm become incorporated into the fibrous connective tissue. The plasma membranes of the fibroblasts are discontinuous where they are adjacent to the fibrous tissue. The fibrils in the connective tissue are obscured by masses of mucopolysaccharide, but there are indications that these fibrils are essentially similar to the intracellular fibrils.
The processes of fibrillogenesis in the moth and in various vertebrate tissues are shown to have many features in common.