Thyroid gland

The thyroid gland forms at week 4 (S10) from an epithelial endodermal prominence just below the tuberculum impar. It penetrates the mesoderm, where it forms two lobes, maintaining contact with its place of origin by means of a pedicle (thyroglossal duct). The thyroid gland grows caudally to the level of the first six tracheal cartilages. At S16, the thyroid gland anlage becomes detached from the pharyngeal floor. In the adult, the point of detachment is visible as the foramen caecum.
Salivary glands
Once the oropharyngeal membrane has disappeared, the border between ectoderm and endoderm is no longer clearly visible. Hence, the dental ridge is now used as a reference line. The labiogingival sulcus (vestibular groove) is to the outside of the dental ridge. The primordium of the ectodermal parotid gland is formed in this sulcus. The linguogingival sulcus is formed to the inside of the dental ridge. The sublingual and submandibular glands, which are formed from the linguogingival groove, are thought to be endodermal in origin.