Abstract
SYNOPSIS. The intraflagellar structure (IFS) of the flagella of Trypanosoma brucei was examined on the basis of ultrathin sections in various planes. The IFS is composed of filaments approximately 50 A thick. These filaments seem to be identical with the protofilaments found earlier to be the basic elements of the contractile flagellar fibrils. The fibrillar system is firmly connected with the IFS and the latter is attached to the flagellar membrane by filaments. The lattice‐like appearance of the IFS is caused by longitudinal and oblique filaments running in different planes. The structure of this network is discussed in detail. The IFS may serve as an abutment for the contractile flagellar fibrils.