In the 4th and 5th week of the development, the formation of the pharyngeal (branchial) arches in the head and neck region contributes greatly to the external appearance of the embryo.
The pharyngeal arches form as the masses of mesenchymal tissue which are invaded by the cranial neural crest cells. Each pharyngeal arch is externally covered by the ectoderm and internally by the endoderm. The pharyngeal arches are separated by deep ectodermal clefts called pharyngeal clefts (grooves). The endoderm of the pharynx, which lines the internal surface of pharyngeal arches, passes into evaginations called the pharyngeal pouches.

1. Pharyngeal arches
2. Lens placode
3. Pericardial swelling
4. Pharyngeal clefts
5. Hand bud

1. External auditory meatus
2. Auditory tube
3. Primary tympanic cavity
4. Cervical sinus
5. Inferior parathyroid gland
6. Thymus
7. Palatine tonsil
8. Superior parathyroid gland
9. Ultimobranchial body

1. Auditory tube
2. Foramen cecum
3. Palatine tonsil
4. Ventral syde of pharynx
5. Tympanic cavity
6. Thyroid gland
7. Ultimobranchial body
8. Foregut
9. Thymus
10. Inferior parathyroid gland
11. Superior parathyroid gland
12. External auditory meatus