Amphioxus pharyngeal development depends on retinoic acid (RA) signaling for both anterior-posterior regionalization and morphogenesis. The anterior portions of larvae at 36 hours post fertilization (hpf) are shown in lateral views (with anterior to the left). The 50-μm scale bar in (A) is applicable to the entire figure. Amphioxus larvae are shown that were injected before fertilization with either 500 μM control antisense morpholino oligonucleotide (MO) or with 500 μM control MO followed by a treatment with 10-7 M RA (at the onset of gastrulation) or with 500 μM Hox1 MO followed by treatment with 10-7 M RA (at the onset of gastrulation). The assayed genes are amphioxus Pax1/9 (A), (A’), (A”), Six1/2 (B), (B’), (B”), Tbx1/10 (C), (C’), (C”), Pax2/5/8 (D), (D’), (D”) and Pitx (E), (E’), (E”).