Histological specimen of a 10-day human embryo represents 2 contacting sacs (amniotic and yolk sacs). Specify the structure that separates the amni-otic cavity from the yolk sac:

Answer options

1. Roof of the yolk sac
2. Embryonic shield
3. Extraembryonic mesoderm
4. Floor of the amniotic sac
5. Amniotic stalk

Explanation

The embryoblast forms an embryonic disc (embryonic shield) which is a bilaminar disc of two layers, an upper layer the epiblast (primitive ectoderm), and a lower layer the hypoblast (primitive endoderm). The disc is stretched between what will become the amniotic cavity and the yolk sac.