Drawings of the cell division patterns during the development of the male gametophyte of M. vestita. (a) Microspores at the time of imbibition. The nucleus is depicted as a black circle, and the plastids are depicted as open ellipses. (b) The gametophyte before the 1st division, when cytoplasmic reorganization occurs. (c) After the 1st mitotic division, a prothallial cell (bottom) is cut off from the rest of the gametophyte and no longer divides. (d) The 2nd division gives rise to 2 antheridial initials, essentially of equal size. (e-g) Each antheridial initial undergoes a series of unequal division that produce the 3 sterile jacket cells. None of the jacket cells can proliferate further. (h) Each primary spermatogenous cell undergoes a symmetric division. (i) The 2 spermatogenous cells in each antheridium undergo a symmetric division to produce 4 spermatocyte mother cells. By this stage, the jacket cells become far less conspicuous and they eventually degenerate. (j) The 4 spermatocyte mother cells in each antheridium undergo a division to produce 8 spermatocytes. At this stage, a blepharoplast (dot) appears in each spermatocyte, and then it rapidly disappears (see Hepler [1976]). Later, the blepharoplasts reform, split and function as the centrosomes for the next mitotic cycle, when each spermatocyte undergoes a division that produces 16 spermatids in each antheridium.