Fig. 1. (A-D) Schematic representation of ascidian heart development. (A) A pair of heart lineage cells (B7.5) in the bilaterally symmetrical 110-cell embryo. (B) Heart progenitors called trunk ventral cells (TVCs) in the tailbud embryo, which differentiate first on each lateral side of the embryo and (C) then fuse ventrally in the swimming larva. In (B,C), upper panels show lateral views and lower panels show ventral views. A pair of anterior muscle cells is also derived from B7.5 (yellow). (D) The heart in the juvenile. (E-H) The developmental fate of the B7.5 blastomere. (E) One of the B7.5 blastomeres is labeled with DiI (arrow). (F) A tailbud embryo and (G) a larva in which DiI was injected into B7.5 at the 110-cell stage. The TVCs are labeled with DiI (arrows), as well as two anterior muscle cells (arrowheads). (H) A juvenile in which DiI was injected into B6.3 at the 32-cell stage. The heart (white arrow), germline cells (black arrow) and degenerated larval tail muscle cells (white arrowhead) are labeled.