Fig. 4. Robust fate specification in nerve cord/notochord precursors. Expression of a notochord marker (Hr-Bra) in 110-cell embryos (A-E), and of a nerve cord marker (Hr-ETR1) in cleavage-arrested 110-cell embryos (A'-E'). Anterior is up. (A-E,A') Vegetal views. (B'-E') Animal views. Manipulation was carried out on the left side of embryos with the exception of those in E and E', where both sides were manipulated. (A,A') 110-cell embryos developed from eight-cell embryos where A4.1 was injected with Hr-FGF9/16/20 MO. Black arrowheads indicate notochord blastomeres that did not express Bra but expressed ETR1. Yellow arrowheads represent notochord blastomeres that weakly expressed Bra. (B,B') a4.2 was injected with FGF mRNA. Red arrowheads represent nerve cord blastomeres that weakly expressed Bra. (C,C') A4.1 and a4.2 blastomeres were injected with FGF MO and mRNA, respectively. ETR1 is expressed broadly in the anterior-animal hemisphere in B' and C'. (D,D') Control embryos to C and C' in which cMO (A4.1) and venus YFP mRNA (a4.2) were injected. (E,E') Endoderm-transplanted embryos. Donor endoderm cells were injected with FGF MO at the eight-cell stage. Black arrowheads indicate nerve cord blastomeres that ectopically expressed Bra. Scale bar: 100 µm.