fig4

Figure 4. Developmental schema of the primate embryonic yolk sac. The primary yolk sac (A) undergoes progressive collapse and remodeling (B), giving rise to the secondary yolk sac (C). The extraembryonic space is populated by Exmes prior to gastrulation. AM: Amnion; pYS: primary yolk sac; sYS: secondary yolk sac; Exmes: extraembryonic mesenchymal cells. Image created by Adobe Illustrator.