An electron microscopic study of extracellular connective tissue fibrils in the chick embryo is reported herein. Primary fibrils (40–45 Å) are first to appear, being present in the primordia of ectodermal boundary (basement) membranes in freshly laid eggs. Free microfibrils (40–150 Å) appear by delamination from boundary membranes from 24 hours incubation onward. The notochord becomes intensely active in forming microfibrils toward the end of the second day. By 72 hours free mesenchymal cells are producing microfibrils. Axial periodicity is acquired relatively slowly by microfibrils in their transition to unit collagen fibrils. At one week's incubation fibrillar diameters and periodic banding still do not match the recognized adult pattern.

The hypothesis that a single developmental line of fibrils leads to all extracellular connective tissue fibrous elements is supported by the ontogeny of fine structure in the chick. Microfibrils first arise in close association with primary fibrils, then from notochordal cells and later, more directly from free mesodermal cells (mesenchyme, fibroblasts). The collagen, reticular and elastic fibers of light microscopy arise from aggregations and alterations of fibrillar material whose line of descent is traceable to the microfibril.