The structure, method of feeding and relationship with the host cells of the tissue parasitizing stages of Histomonas meleagridis have been studied by means of light microscopy, cytochemistry and electron microscopy. The invasive stage is amoeboid, lacks flagella but has what appears to be remnants of the flagella apparatus, and feeds by phagocytosis. The vegetative stage is a round quiescent form. It apparently feeds by secreting proteolytic enzymes, which carry out extra-corporeal digestion of the host's tissues, and then taking up small particles of disrupted host cells by means of pinocytosis and probably by diffusion. The vegetative stage also contains remnants of flagella apparatus. Neither the invasive stage nor the vegetative stage contains mitochondria and succinic dehydrogenase is absent. It is concluded that there is no resistant stage.
The reaction of the host's tissues to the parasites has been briefly described.
The authors are grateful to Dr C. C. D. Shute for permission to use the electron microscope, to Mrs B. Fisher and Mr M. Shirley for technical assistance and to Mr P. Rogers for assistance with the photography.
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