l62 



THE ENTODERMAL CANAL AND THE BODY CAVITIES 



A mound of lymphoid tissue presses against the epithelium of the tonsillar fossa in 

 140 mm. (C R ?) fetuses and forms the palatine tonsil. The lymphocytes are probably of 

 mesodermal origin (Hammar, Maximow). 



Imperfect closure of the branchial clefts (usually the second) leads to the formation of 

 cysts, diverticula, or even of fistulae. According to Hammar (Arch. f. mikr. Anat., Bd. 61, 

 1903), the lateral pharyngeal recess (of Rosenmiiller) is not a persistent portion of the second 

 pouch as His asserted. 



A subepithelial infiltration of lymphocytes during the sixth month gives rise to the 

 median pharyngeal tonsil, which like the lingual tonsil is not of pharyngeal pouch origin. 

 Immediately caudad is a recess, the pharyngeal bursa, formed by a persistent connection of 

 the epithelium with the notochord (Huber). 



THE THYMUS 



The thymus anlage appears in 10 mm. embryos as a ventral and medial 

 prolongation of the third pair of pouches (Figs. 168 and 169). The ducts con- 

 necting the diverticula with the pharynx soon disappear so that the thymus an- 

 lages are set free. At first hollow tubes,- they soon lose their cavities and their 



Foramen azcum, 



Palatine tonsil 

 Epilhelial bodies 



Thyreoglossal duct 



Thyreoid anlage 

 Thymus anlages I _ " '^Trachea 

 Posl-branchial body 



Fig. 169. — Diagram in ventral view of the pharynx and pharyngeal pouches, showing the origin 

 of the thymus and thyreoid glands and of the epithelial bodies (modified after Groschuff and Kohn). 

 I-V, first to fifth pharyngeal pouches. 



lower ends enlarge and migrate caudally into the thorax, passing usually ven- 

 tral to the left vena anonyma. Their upper ends become attentuate and atrophy, 

 but may persist as an accessory th3Tnus lobe (Kohn). The enlarged lower ends of 

 the anlages form the body of the gland, which is thus a paired structure (Fig. 170). 

 At SO mm. (C R) the thymus still contains solid cords and small closed vesicles of 

 entodermal 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, 



