THE EPITHELIAL BODIES OR PAEATHYEEOIDS 1 63 



after which its degeneration begins. This process proceeds slowly in healthy 

 individuals, rapidly in case of disease. The th3Tnus may function normally 

 until after the fortieth year. 



The ventral diverticulum, of the fourth pouch is a rudimentary thymic anlage. 

 It usually atrophies. 



It is now generally believed that the entodermal. epithelium of the thymus is converted 

 into reticular tissue and thymic corpuscles. The "lymphoid" cells are regarded by Hammar, 

 Maximow, and recently by Badertscher (Amer. Jour. Anat., vol. 17, 1915) as immigrant 



Jugular Carotid Carotid 



vein artery artery 



Tn ntar vein 



Thyreoid - -^ — -^— ^ « P ' l r / Thyreoid 



Parathyreoid IV. — V ■ — ^ ' ]r^ — Parathyreoid IV. 



Parathyreoid III, ^^ L^ 1 Parathyreoid III. 





Thymus -^ \^ \ -^^ — ^^ Thymus 



Superior vena cava 



X, y -^ _^ 



Fig. 170. — Reconstruction of the thymus and thyreoid glands in a 26 mm. human embryo (after 



Tourneaux and Verdun). X 15. 



lymphocytes derived from the mesoderm. According to Stohr, they are not true lympho- 

 cytes, but are derived from the thymic epithelium. WeiU (Arch. f. mikr. Anat., Bd. 83, 1913) 

 has observed the development of granular leucocytes in the human thymus gland. 



THE EPITHELIAL BODIES OR PARATHYREOIDS 



The dorsal diverticula of ithe third and fourth pharyngeal pouches each give 

 rise to a small mass of epithelial cells termed an epithelial body (Fig. 169). Two 

 pairs of these bodies are thus formed, and, with the atrophy of the ducts of the 

 pharyngeal pouches, they are set free and migrate caudalward. They eventually 

 lodge in the dorsal surface of the thyreoid gland, the pair from the third pouch 

 lying one on each side at the caudal border of the thyreoid in Kne with the thymus 

 anlages (Fig. 170). The pair of epithehal bodies derived from the fourth pouches 

 are located on each side near the cranial border of the thyreoid. From their 

 ultimate relation to the thyreoid tissue the epithelial bodies are often termed 

 parathyreoid glands. The soKd body is broken up into masses and cords of poly- 

 gonal entodermal cells intermingled with blood vessels. In postfetal life, lumina 

 may appear in the cell masses and fill with a colloid-like secretion. 



