A previously unsuspected cytoplasmic organoid, fibrillar in nature, was discovered by Bessis and Breton-Gorius. This structure exhibits a very high degree of structural organization, and appears as a barrel-shaped fibrillar formation, opened at both ends and enclosing an associated granular component within. Comparable fibrillar formations are rarely found in normal cells, even in the simplest form of a few closely packed fibrils. In its highest degree of structural organization, the whorl, the fibrillar formation has been found only in the blast or promyelocytes stages. Although the overall dimensions of this structure are well within the resolving limits of the light microscope, it is not visible in fresh, Wright stained or Giemsa stained preparations.