Germ layers

In the 3rd week of pregnancy an early placenta develops and the embryo divides into three germ layers: ectoderm, endoderm and mesoderm. These three layers are positioned as flat oval shaped disks. The outermost layer, ectoderm, will eventually differentiate to become the central nervous system and skin. The middle layer, mesoderm, will differentiate to become connective tissues, muscles, bones, circulatory system and the urogenital system. The innermost layer, the endoderm, gives rise to the gastrointestinal (GI) tract and the respiratory system (Rubarth & van Woudenberg, 2016).