1. The morphology and life-history of Dientamoeba fragilis, as observable in cultures, are described and illustrated, and the stages correlated with those commonly found in human faeces.
2. The nuclear structure especially is reinterpreted, and nuclear and cytoplasmic division are described in detail.
3. It is shown that the “normal” binucleate condition of this “amoeba” represents an arrested telophase stage of mitosis—the “granules” in the nuclei being really chromosomes (constant in number), and the strand connecting the nuclei being a persistent centrodesmus. The chromosome number is probably 6 (certainly not 4).
4. Binary fission is the only method of reproduction hitherto observed: no cysts or other stages in the life-history have been discovered.
5. Attempts to transmit D. fragilis to a man and 2 macaques (M. rhesus and M. sinicus) by administration of trophic amoebae per os—and in the case of M. rhesus inoculation per anum—were unsuccessful. It was also found impossible to infect chickens permanently by injection per anum (though one chick acquired an infection lasting for a week).
6. Consideration of its cytology and development leads to the conclusion that D. fragilis is not a true amoeba but an aberrant flagellate closely related to Histomonas. No flagellate stages, however, have yet been discovered.
7. Some consequences of this conclusion are briefly discussed, and hopeful directions for further inquiries are indicated.
8. On analogy with Histomonas, it is suggested that D. fragilis may be conveyed from man to man not by direct oral infection with trophic forms but in the ova of a nematode worm—possibly Trichuris (or Ascaris?).