What is the process of differentiation in embryonic development?

I- The internal mass of the blastocyst divides and forms a new cavity, called the secondary or definitive vitelline cavity.

II- The contact zone between the two formed vesicles constitutes what is called the embryonic area or blastodisc.

III- Only the double layer of cells that make up the embryonic disc is destined to form the new individual, the upper layer of cells is called ectoderm and the lower endoderm, and between these two a third layer of cells called mesoderm develops.

IV- The multiple pregnancy polizygotic is produced by the process of polio.