The respiratory primordium buds off the primitive foregut and grows caudally, on a lengthening stalk; the cephalic end of the stalk develops into the glottis and infraglottis, and the rest becomes the trachea. Compression by pharyngeal mesoderm cephalic to the respiratory diverticulum obliterates the foregut lumen ventrodorsally as far cranially as the 4th pharyngeal pouches, forming an epithelial lamina with a narrow pharyngoglottic duct along its dorsal border. The mesoderm also raises an epiglottic and two arytenoid swellings in the pharyngeal floor at the level of the 4th pouches; the triangular 'cecum', bounded by these swellings, grows caudally along the ventral border of the epithelial lamina to just above the glottis. Beginning at stage 21, the epithelial lamina separates cephalocaudally, bringing the cecal lumen into continuity with the pharyngoglottic duct to form the laryngeal vestibule; when the separation is complete, the vestibule is continuous with the infraglottic cavity. Meanwhile, bilateral pouches arising from the caudal end of the cecum form the ventricles, the lower lips of which become the vocal folds, and the pharyngeal mesoderm surrounding the laryngeal cavity gives rise to the laryngeal cartilages and intrinsic musculature. The cricoid chondrifies bilaterally from a single center in the ventral arch of a precartilaginous template that encircles the infraglottic cavity, and on meeting forms the dorsal lamina. Each arytenoid chondrifies from a single center, and each half of the thyroid cartilage chondrifies from two. Anlagen for the intrinsic muscles appear during stage 17, sites of individual muscles are recognizable by stage 23, and myofibrils are present by the 12th week.