Fig. 1. Schematic of primate peri-implantation development. (A) A blastocyst-stage primate embryo showing the trophectoderm (red) and inner cell mass (ICM, orange). (B) A portion of a postimplantation day 10, rhesus macaque embryo, showing the development of the amnion (blue) through the cavitation of the ICM (orange), together with the trophectoderm (red) and extraembryonic endoderm (green). (C) A postimplantation day 9, human embryo, showing the extension of the extraembryonic endoderm (green) around the blastocoel cavity. (D) A postimplantation day 10.5, human embryo, showing the characteristic mesh of extraembryonic endoderm within the blastocoel cavity (green). Maternal tissue is shown in purple. (E) A day 11.5 human embryo, showing the extraembryonic mesoderm in yellow. (F) A portion of a day 13, rhesus macaque embryo, showing the formation of the primitive streak (light orange) from the caudal epiblast (dark orange), subsequent to the extensive development of the extraembryonic mesoderm (yellow). The secondary yolk sac is shown in green. Data from Enders et al., Hamilton and Luckett (Enders et al., 1986; Hamilton, 1972; Luckett, 1978).