The amnion and chorion are composed of extraembryonic ectoderm and mesoderm, whereas the yolk sac is composed of extraembryonic endoderm and mesoderm. By day 13, the connecting stalk, a dense portion of extraembryonic mesoderm, restrains the embryonic disc in the gestational sac.
The amnion and chorion are made from extraembryonic ectoderm and mesoderm, while the yolk sac is composed of extraembryonic endoderm and mesoderm. By day 13, the connecting stalk, a dense section of extraembryonic mesoderm, keeps the embryonic disc within the gestational sac.