The dermis is formed by the endoderm at the top of the yolk sac and the ectoderm at the bottom of the amniotic sac.
1. Endoderm at the top of the yolk sac: When the embryonic cells, i.e. the inner cells, divide and proliferate towards the side of the blastocyst cavity, the neat, cubic layer of cells formed is the endoderm. With the passage of time, the proliferating cells of the endoderm grow downward, enclosing a small sac called the yolk sac, and the top of the sac is the endoderm in the dermisphere.
2. Ectoderm at the base of the amniotic sac: As the endoderm forms, the remaining inner cells above it rearrange themselves to form a layer of columnar cells called ectoderm.
Over time, the trophoblast on the back of the ectoderm begins to divide and proliferate, resulting in the formation of flat amniotic cells, whose circumference connects to the ectoderm and together they enclose the amniotic sac, with an internal cavity, the amniotic cavity, at the bottom of which is the ectoderm in the diamniotic layer.