Medical Definition of promastigote
: the motile, elongated, extracellular form in the life cycle of some protozoans (family Trypanosomatidae, and especially genus Leishmania) that is characterized by a single anterior flagellum and no undulating membrane In the sandfly gut the amastigotes leave the host cell, transform into promastigotes, and replicate extracellularly.—David J. Wyler and Anthony R. Mattia, The New England Journal of Medicine, 17 Mar. 1994
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