Electron microscopy has clarified the structure and taxonomic position of the genus Mesostigma (type species M. viride Lauterborn) by showing that the two unequal flagella are covered with small scales of a characteristic shape and that they arise from a conical pit in the upper surface of the cell which is lined by a dispersed system of tubes or fibres as in the related genus Pyramimonas. The periplast over the rest of the cell is shown to contain two other very distinct kinds of scales which are described. The origin of all three types of scale has been traced to vesicles derived from Golgi cisternae, and scale liberation though not traced in detail is believed to take place into the flagellar depression. The peculiar flattened shape of the chloroplast with restriction of starch storage to a swollen border, the structure of the pyrenoid and the distribution within the cell of other cell components is also indicated in outline. It is concluded that though unique in many respects the genus Mesostigma is unequivocally a member of the Prasinophyceae (sensu Parke & Manton, 1965) and is relatively remote from the Volvocales in which it was formerly classed.
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Manton et al. (1965) studied this question.