A symmetrical, extracellular fibril.

Symmetrical, extracellular fibrils, which are related to the "special fibrils" of the dermis described by Palade and Farquhar, have been found along the outer surface of the basement membrane covering the notochord in the tail of Rana catesbeiana (bullfrog) tadpoles. The fibrils are approximately 7,500 A long and occur singly or in clusters. The single fibrils are characterized by a symmetrical transverse band pattern and by attachment at both ends to the basement membrane. The clusters are various complex configurations which seemingly represent symmetrical fibrils in different states of aggregation. Symmetrical fibrils also occur in the skin of the tadpole tail and in the skin of the toad, Bufo marinus. It is proposed that a narrow, symmetrical fibril is the fundamental "special fibril."