B Breakdown of the thyroglossal duct
The thyroid is the first endocrine gland to appear in embryonic development, during week 4. The endoderm of the floor of the primitive pharynx thickens in its medial part, with a downgrowth giving origin to the thyroid diverticulum. The developing thyroid continues its descent, reaching the level of the hyoid bone by week six. At this point the thyroid is connected to the tongue by the thyroglossal duct, which opens in the tongue at the foramen cecum. The thyroid diverticulum is originally hollow, but soon becomes solid and divides. By week seven, the thyroid reaches its final position, and the thyroglossal duct starts to degenerate. The thyroid primordium starts to breakdown into a network of epithelial cords or plates, due to the invasion of the surrounding vascular mesenchyme. By week ten, the cords divide into small cellular groups, and then a lumen forms in each of these. The cells organize themselves into a single layer, and by week eleven colloid begins to appear within these structures, the thyroid follicles. By week twelve, thyroid hormones are being synthesized.