ngitudinal section of vegetative colony. (E) Eyespot composed three layers of globules. Using TEM on vegetative cells, T. socialis cellular structure was very similar to that of a vegetative C. reinhardtii cell [13] ; with a cup-shaped chloroplast occupied the peripheral region of the protoplast and a nucleus was centrally located ( Figure 6D ). The chloroplast contained a large pyrenoid in the bottom and an eyespot in the anterior periphery, again similar to C. reinhardtii cells and supporting the close evolutionary relation of the two species [9] , [10] , [15] . In T. socialis , the eyespot was concave, and composed of two or three layers of electron-dense globules lying just beneath the chloroplast membrane ( Figure 6E ). The innermost of the three layers was often discontinuous in section. Discussion Asymmetrical Cells and Cytoplasmic Bridges between Daughter Protoplasts in the Volvocine Algae The present immunofluorescence microscopic observations clearly demonstrated that the four-celled colony of T. socialis had cells with rotational asymmetry and separated BB ( Figures 3 ). These two situations are essentially the same as those of other multicellular volvocalean species of the Goniaceae and Volvocaceae, G. pectorale (figures 31–33 [24] ), Astrephomene gubernaculifera [30] , Platydorina caudata [31] , and V. carteri [32] . In contrast, cells of C. reinhardtii exhibit rotational symmetry in arrangement of MTR and have adjacent BB ( Figure 3 ) as previously described by Ringo (figures 13 and14 [23] ) and Preble et al. [33] . Previous studies demonstrated that peripheral cells of G. pectorale and cells of V. carteri beat two flagella in nearly the same direction so that they can swim effectively as cooperative multicellular organisms, whereas in the unicellular species C. reinhardtii cells beat their flagella like breast stroke so that the unicells can swim effectively [34] , [35] . Rotational asymmetry of MTR in cells might be important for multicellularity in