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Fig. 1. A binary choice between notochord and neural fates in ascidian
embryos. All embryos are viewed from the vegetal pole with the anterior
side up. Because ascidian embryos are bilaterally symmetrical, pairs of
blastomeres will be referred to by their blastomere name (e.g. A6.4), rather
than as `a pair of'. A4.1 (green), located in an anterior-vegetal position of
the eight-cell-stage embryo, divides twice to generate four cells (outlined by
green dotted line) in the 24-cell-stage embryo. These four cells consist of
two mother cells of notochord/neural precursors, named A6.2 and A6.4 (red),
one endoderm/mesenchyme mother cell and one endoderm precursor. From the 24-
to 32-cell stages, no cell divisions take place in the A4.1-lineage. At the
late 32-cell stage, the cells in the A4.1 lineage enter mitosis and each of
the A6.2 and A6.4 mother cells divides along the anterior-posterior axis to
give rise to one notochord (orange) and one neural (yellow) precursor at the
44-cell stage. Each of the precursors then divides in the medial-lateral
direction to generate four notochord and four neural precursors in the
110-cell-stage embryo. Therefore, the A4.1 cell generates four notochord and
four neural precursors at the 110-cell stage, whereby each of the A6.2 and
A6.4 mother cells gives rise to two notochord and two neural precursors. The
anterior-animal cell, a4.2, of the eight-cell-stage embryo, and its
descendents, are coloured in blue.
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