Answer the following question given this paragraph: 

The embryo, which begins as a flat sheet of cells, begins to acquire a cylindrical shape through the process of embryonic folding (Figure 28.14). The embryo folds laterally and again at either end, forming a C-shape with distinct head and tail ends. The embryo envelops a portion of the yolk sac, which protrudes with the umbilical cord from what will become the abdomen. The folding essentially creates a tube, called the primitive gut, that is lined by the endoderm. The amniotic sac, which was sitting on top of the flat embryo, envelops the embryo as it folds.


Q: What sac, which was sitting on top of the flat embryo, envelops the embryo as it folds?

 Choices:

- uterus

- umbilical sac

- epithelial sac

- amniotic sac

A:
amniotic sac
