Fig. 2From: Embryogenesis of flattened colonies implies the innovation required for the evolution of spheroidal colonies in volvocine green algaeSuccessive cell divisions and expansion of the cell layer in embryogenesis of G. pectorale. Successive stages of an embryo observed by time-lapse analysis from the anterior-lateral view (Additional file 2). All images are at the same magnification. Scale bars: 5 μm. Note the longitudinal axis of each daughter protoplast, indicated by the positions of apical ends (arrowheads) and chloroplasts (letter c). a Early 8-celled stage. b Late 8-celled stage. c Early 16-celled stage. Rotation of daughter protoplasts is not observed during cell divisions (a–c). d Mid 16-celled stage during partial inversion. The cup-shaped cell layer of the embryo has expanded and the apical ends of outer daughter protoplasts (arrowheads) have separated from one another and moved outwards. e Late 16-celled stage, just prior to hatching. The cell layer has increasingly expanded and rotated slightly to show an almost anterior viewBack to article page