The development of the egg of Polymorphus minutus, an acanthocephalan parasite of water birds, is described. It is compared with the published descriptions of the embryology of the only other two species of the phylum which have so far been studied. The three species represent each of the three groups of the phylum, and this allows an overall assessment of the peculiarities of embryonic development in the phylum to be made. It is shown that it is very unusual, the polarity of the egg being reversed, the first two cleavages being more or less parallel, the cellular boundaries disappearing at an early stage and most of the nuclei condensing and forming a compact central mass.