THE PATHOLOGICAL ANATOMY 297 



squamous cell epithelium and (2) those lined with ciliated epithelium. 

 As the upper part of the thyroglossal duct usually has squamous epithe- 

 lium, and the lower part ciliated, the type of cyst indicates roughly 

 its origin. In many of the cases with thyroglossal cysts or with per- 

 sistence of the thyroglossal tract, the hyoid hone contains a fora- 

 men through which the tract passes. This is due to the hyoid de- 

 veloping much later in embryonic life than the thyroid, and when the 

 tract persists the hyoid is developed around it. Accessory or aberrant 

 thyroid tissue, apart from that described above in relation to the thyro- 

 glossal tract, is most frequently seen in the region of the ascending and 

 transverse portions of the aortic arch. Occasionally, thyroid fragments 

 are present in the thymus, at the bifurcation of the trachea and as far 

 down as the esophageal opening in the diaphragm. In the large terato- 

 matous growths of the branchial cleft and ovarian regions thyroid tissue 

 is frequently present. Many instances of the occurrence of thyroid tissue 

 in ovarian tumors and cysts have been reported. The cystic spaces of such 

 tumors and cysts, filled with desiccated albuminous material, often re- 

 semble thyroid alveoli and are only differentiated from thyroid by a test for 

 iodin. Bauer, E., Erdheim, S., Erdheim, J. (fr), Kanthack, Kadyi, Made- 

 lung, Marshall (a.) (fr), Schwartz and Thompson, Ungermann, Schilder. 



