172 THE ENTODERMAL CANAL AND ITS DERIVATIVES 



At 50 mm. the thymus still contains solid cords and small closed vesicles of ento- 

 dermal cells. From this stage on, in development, the gland becomes more and 

 more lymphoid in character. Its final position is in the thorax dorsal to the cranial 

 end of the sternum. It grows under normal conditions until puberty, after which its 

 degeneration begins. This process proceeds slowly in healthy individuals, rapidly 

 in case of disease. The thymus may function normally until after the fortieth year. 



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 

 and Maximow as immigrant lymphocytes derived from the mesoderm. According to Stohr, 

 they are not true lymphocytes but are derived from the thymic epithelium. Weill (Arch. f. 

 mikr. Anat., Bd. 83, pp. 305-360) has observed the development of granular leucocytes in the 

 human thymus gland. 



THE EPITHELIAL BODIES OR PARATHYREOIDS 

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

 rise to a small mass of epithelial cells termed an epithelial body (Fig. 163). 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 line with the thymus 

 anlages (Fig. 164). The pair of epithelial 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 solid body is broken up into masses and cords of poly- 

 gonal entodermal cells intermingled with blood-vessels. In post-fetal life, lumina 

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



The ventral diverticulum of the fourth pouch is a rudimentary thymic an- 

 lage. It soon atrophies. 



The ultimobranchial body is the derivative of the fifth pharyngeal pouch 

 (Fig. 163). With the atrophy of the duct of the fourth pouch it is set free and 

 migrates caudad with the parathyreoids. It forms a hollow vesicle which has 

 been termed the lateral thyreoid. According to Grosser (Keibel and Mall, vol. 

 2, p. 467) and Verdun, it takes no part in forming thyreoid tissue but atrophies. 

 The term lateral thyreoid when applied to it is therefore a misnomer. 



THE THYREOID GLAND 

 The thyreoid anlage (Fig. 163) is bilobed before the thyreoglossal duct dis- 

 appears. It soon loses its lumen and breaks up into irregular solid cords of tissue 



