Foregut

The primary mouth (stomatodeum), which is covered with ectoderm, continues then into the foregut, which is lined with endoderm. Rostral and caudal parts of the foregut can be distinguished.The rostral part (embryonic pharynx) extends from the oropharyngeal membrane to the pulmonary bud. The caudal part (postpharyngeal foregut) extends to the hepatic bud. The oesophagus, the stomach and the rostral part of the duodenum all develop from this part of the foregut. The oropharyngeal membrane ruptures and disappears completely by S11.

Pharynx

Following cephalic folding during weeks 4 and 5, the lateral parts of the pharynx undergo a secondary segmentation. This leads to the formation of four pharyngeal arches. Each arch contains an aortic arch (pharyngeal arch artery), a skeletal element (from mesenchymal condensations or blastemas), a muscle primordium and a motor nerve. The arches are bounded towards the inside (endoderm) by the pharyngeal pouches and towards the outside (ectoderm) by the pharyngeal clefts. The four pharyngeal arches are expanded by the presence of the aortic arches. The corresponding motor nerves are: 1st arch (mandibular arch), mandibular nerve; 2nd arch (hyoid arch), facial nerve; 3rd arch, glossopharyngeal nerve; 4th arch, vagus nerve. The mesenchyme of the pharyngeal arches is formed from paraxial unsegmented mesoderm, which is thought to be derived from the primitive node at a very early stage. The mesenchyme is later supplemented with material from the neural crest.