After implantation of the blastocyst the embryonic cells divide further. At the end of week 2, three germ layers are formed. The endoderm is the innermost layer of the early embryo: ectoderm – mesoderm – endoderm. The endoderm is the base of the gastrointestinal system (pharynx, oesophagus, stomach, small and large intestine, liver, pancreas and bladder) and also for the lungs and thyroid glands  (Hill, 2020).