Animal, o-iropa, seed); that is to say, cells capable of motility through the agency of cilia.
Such ciliary motion is known in the adult condition of the cells of Volvocaceae, but where this is not the case the reproductive cells are endowed with motility for a brief period.
Thus, although isogamy consists in typical cases of a union of naked motile gametes by a fusion which begins at the beaked ends, and results in the formation of an immotile spherical zygote surrounded by a cell-wall, in Leptosira it is noticeable that the fusion begins at the blunt end; in a species of Chlamydomonas the two gametes are each included in a cell-wall before fusion; and in many cases the zygote retains for some time its motility with the double number of cilia.
Aplanospores would seem to represent zoospores arrested in their development; without reaching the stage of motility, they germinate within the sporangium.
The experiment of Engelmann referred to deserves to be mentioned here, if only in illustration .of the use to which algae have been put in the study of physiological problems. Engelmann observed that certain bacteria were motile only in the presence of oxygen, and that they retained their motility in a microscopic preparation in the neighbourhood of an algal filament when they had come to rest elsewhere on account of the exhaustion of oxygen.