Stylized sequences of the different inversion processes in Volvocaceae: type A and type B. Schematic representation of midsagittal cross sections of embryos showing inversion of type A and type B inverters. (A) Type A inversion in Volvox carteri. (A1) Before the initiation of inversion. (A2) Inversion begins with the appearance of a swastika-shaped opening, the phialopore, at the anterior pole. (A2 to A8) The cell monolayer curls outward and backward over the simultaneously contracting posterior hemisphere and the phialopore widens; the posterior hemisphere moves toward the phialopore; the region of maximum cell-sheet curvature, the bend region, moves progressively to the posterior pole. (A9,A10) Inversion ends with the closure of the phialopore. (B) Type B inversion in Volvox globator. (B1) Before the initiation of inversion. (B2) Inversion begins with the contraction of the posterior hemisphere. (B3) Contraction proceeds and accompanies movement of the posterior hemisphere into the anterior hemisphere; this invagination of the cell sheet generates a ring of maximum curvature between the two hemispheres, the bend region. (B4,B5) The posterior hemisphere lies completely within the anterior hemisphere but moves further still in direction of the anterior pole; a circular opening, the phialopore, appears at the anterior pole and widens. (B5 to B7) The widening of the phialopore proceeds and the anterior hemisphere moves over the already inverted posterior hemisphere. (B7,B8) The last third of the anterior hemisphere moves over the already inverted cell monolayer. (B9,B10) Inversion ends with the closure of the phialopore. In both species, the side of the cell layer that is outside in the adult configuration and from which the flagella will emerge is presented in brown, and the side of the cell layer that is inside in the adult configuration and where the reproductive cells are located, is presented in green.