Figure 1.

(a) The trilaminar embryonic disc in the context of the amnion and the yolk sac. Red, ectoderm; blue, mesoderm; green, endoderm; sm, stomatopharyngeal membrane; Hn, Hensen's node and ps, primitive streak. The ectoderm has been removed in the region of the stomatopharyngeal membrane to show the cardiac crescent. The adjacent pharyngeal mesoderm is central relative to the crescent, while the mesoderm of the septum transversum is peripheral; these relationships are shown in detail in (b). In panel (c), we provide a detail of the cardiogenic region, and suggest a hypothetical fate for the different cardiac regions, largely based on the previous work of Stalsberg & de Haan (1969).