A 28-year-old primigravida presents at 20 weeks gestation for routine ultrasound. The scan reveals a large cystic mass in the fetal neck with multiple septa, extending from the angle of the mandible to the clavicle. The mass does not enhance significantly on Doppler. Amniocentesis and karyotyping are normal. The obstetrician counsels the parents that this lesion arises from remnants of the second and third pharyngeal pouches. Which of the following structures is derived from the same germ layer as the epithelial lining of this cyst?