Fig. 3. AP-2 expression in amphioxus. (A) Cross-section through the 9 hour neurula depicted in B at position a, showing AP-2 transcripts in the non-neural ectoderm. (B) Dorsal anterior view of 9 hour neurula showing exclusion of AP-2 from the neural plate. (C) Dorsal view of 11.5 hour neurula, anterior is towards the left. AP-2 is expressed in the epidermis overgrowing the neural plate. (D) Cross-section through the embryo in C at position d. (E) Side view of 18 hour neurula. AP-2 expression in the epidermis is being extinguished from the anterior- and posterior-most ends of the embryo. (F) Optical horizontal section of 24 hour embryo through the gut with anterior towards the top. A spot of AP-2 positive cells is apparent in the left gut diverticulum, the endodermal portion of the pre-oral pit (arrow). (G) Cross-section of 9 hour neurula in C at position g. At more posterior levels, the AP-2 positive epidermis has completely covered the open neural plate. (H) Side view of 24 hour embryo with anterior to the left. AP-2 expression is seen in the cerebral vesicle and pre-oral pit, but has largely faded from the epidermis. (I) Optical horizontal section of 24 hour embryo in H at the level of the neural tube showing symmetrical AP-2 staining in the cerebral vesicle. Anterior is towards top. (J) Two-day larva with AP-2 expression persisting in the cerebral vesicle and pre-oral pit. (K) 36 hour larva with AP-2 expression in cerebral vesicle and pre-oral pit. (L) Cross-section through 24 hour embryo in H at position 1. AP-2 expression in the cerebral vesicle is limited to ventrolateral levels. ar, archenteron; ect, non-neural ectoderm/epidermis; np, neural plate; n, notochord; cv, cerebral vesicle; pp, pre-oral pit; ld, left gut diverticulum.