356 TEXT-BOOK OF EMBRYOLOGY. 



other abdominal organs directly or indirectly to the persistence of the right 

 venous trunks. Certainly a reversal in the position of the stomach would 

 cause a reversal in the position of the duodenum. 



If these conditions are the real ones, the fact that the thoracic organs can be 

 transposed without a transposition of the abdominal organs, or vice versa, 

 is accounted for. The primary bend of the heart tube occurs at a very early 

 period, before the changes in the vessels in the region of the liver. Conse- 

 quently a reversal of the conditions of the omphalomesenteric at a very early 

 stage only would be likely to affect the heart. The principal changes in size 

 of the venous trunks in the abdominal region take place after their channels 

 have been broken up in the liver. In other words, the modifications in the veins 

 in the liver occur after the definite relations of the heart have been established. 

 Therefore the transposition of the abdominal organs may take place after the 

 heart has begun to develop normally. 



THE MOUTH. Anomalies in the mouth region, due to defective fusion of 

 the processes that bound it, have been considered elsewhere (p. 212). 



Anomalies of the tongue sometimes arise as the result of imperfect develop- 

 ment of one or more of its anlagen. Imperfect development of the tuberculum 

 impar results in total or partial lack of the anterior part. Defects in the root 

 are probably due to imperfect development of one or both of the paired anlagen 

 (p. 320). Malformations of the lower jaw (micrognathus, agnathus) are 

 usually accompanied by malformations of the tongue, both structures being 

 derived largely from the first pair of branchial arches. 



THE PHARYNX. The pharynx is the seat of cysts, fistulae and diverticula 

 which have been considered in connection with the anomalies in the region of 

 the branchial arches and grooves (Chap. XIX) . 



The ihyreoid gland is not infrequently the seat of certain anomalies that 

 arise as the result of abnormal development. Persistent portions of the thyreo- 

 glossal duct, the upper end of which is indicated by the foramen cascum linguae, 

 may give rise to cystic structures extending to the region of the hyoid bone. 

 Persistent portions of the duct may even give rise to accessory thyreoid (supra- 

 hyoid, prehyoid) glands (p. 332; also Fig. 298). Considerable variation also 

 exists in the isthmus and lateral lobes of the thyreoid, due to variation in the 

 manner of development of the medial anlage. 



Impaired development of the thymus gland sometimes leads to cysts which 

 come to lie in the anterior mediastinum. 



THE (ESOPHAGUS. Very rarely the oesophagus is entirely lacking, being 

 represented by a mere cord of tissue. More frequently it is defective in certain 

 parts. The atresia may begin just below the pharynx or just above the stomach, 

 the intermediate portion being composed of a cord of fibrous tissue. Occasion- 

 ally the non-atretic portion opens into the trachea. Possibly this represents 



