Fig. 5. Mitosis in 2 cell embryos. (A,B) Interphase in a 2 cell embryo. Cortical MT arrays are reformed in both the thallus and the rhizoid. (A) Projection of all sections, (B) single equatorial optical section. MTs connect the thallus nucleus to the upper cell cortex. MT arrays in the rhizoid are detected in the same plane. Inset shows the rhizoid nucleus with two centrosomes that separate around the nucleus. (C,D) Mitosis entry in a 2 cell embryo: (C) A few cortical MTs probably connected with nuclei are seen in both the thallus and the rhizoid cell. (D) One nuclear section in the thallus. A bundle of MTs connects one centrosome to the injection site. The other centrosome is connected through MT bundles to the cortex of the embryo (see C). (E) The rhizoid nucleus is rotating. Both centrosomes separated around the nuclei are connected to the rhizoid apex. (F) 3 hours later cortical MTs are reduced to star shaped structures and metaphase spindles are seen in both the rhizoid and the thallus cells (G). Note that the thallus spindle is perpendicular to the growth axis whereas the spindle in the rhizoid is not yet aligned with this axis (projection of all sections). Bar, 15 µm.