Figure 1.
Extraembryonic tissues during amniote development. (A) A simplified phylogenetic tree of the extant amniotes. a, amniotes; m, mammals; r, reptiles (including birds). (B) A developing turtle (Pelodiscus sinensis) embryo, showing the embryo proper together with all associated extraembryonic tissues. (C) Two examples of how an amniote embryo organizes its extraembryonic tissues. Modified after Ferner and Mess,3 with permission from Elsevier. Left: schematic view of the organization in birds and many reptiles. Right: schematic view of the organization in eutherian mammals. 1, amnion; 2, chorion; 3, yolk sac; 4, allantois. Germ layers are color-coded (green, ectoderm; red, mesoderm; yellow, endoderm).