Fig. 2. Formation of the microtubule (MT) array. (A-C) Vegetal views of microtubule array formation during the first cell cycle. Times shown are normalized times (NT), with 0.0 NT representing fertilization and 1.0 NT representing the first cleavage division. (A) At 0.4 NT, short disorganized MT polymers have started to appear in the vegetal shear zone. (B) By 0.5 NT, more MTs are present, but are not yet aligned. (C) By 0.7 NT, during peak cortical rotation, the vegetal shear zone is populated by a parallel array of MT bundles that are aligned along the axis of rotation. (D-F) MTs of the vegetal array arise from several sources. (D) The sperm centriole introduces polarity by acting as a minus-end MT-organizing center (-). The resulting radial array of MTs is called the sperm aster. (E) MTs from the sperm aster grow toward the periphery of the egg, as do additional MTs from unknown sources in the core cytoplasm. In addition, short disorganized MT polymers arise in the vegetal shear zone. (F) During rotation, MTs from deep in the cytoplasm bend into the vegetal shear zone and align with peripheral MTs to form the parallel array, with the plus-ends (+) of the growing MTs pointing towards the future dorsal (D) side of the embryo. V, ventral. (A-C) Reproduced, with permission, from Cha and Gard (Cha and Gard, 1999). (D-F) Adapted, with permission, from Houliston and Elinson (Houliston and Elinson, 1991b).