After a period of growth, to 4 pm at the blastoderm over the weekend inside and outside the two germ layers, from the ectodermal layer of differentiated cells within the mesoderm to form embryos.

• 经过一段时间的发育，到4周末时在胚盘内、外两胚层之间，由外胚层分化出一层细胞，形成胚内中胚层。