164 



THE ENTODERMAL CANAL AND THE BODY CAVITIES 



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 protracted connection 

 of the epithelium with the notochord (Huber). It bears no relation to Seesel's pouch. 

 According to Hammar, the lateral pharyngeal recess of (Rosenmuller) is not a persistent 

 portion of the second pouch, as His asserted. 



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

 the formation of cysts, diverticula, or even fistulas. 



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 

 connecting the diverticula with the pharynx soon disappear so that the 

 thymus anlages are set free. At first hollow tubes, they soon lose their 

 cavities and their lower ends enlarge and migrate caudally into the thorax, 



Foramen ciecum 



Palatine tonsil 

 Parathyreoid anlages 



Thyreoglossal dud 



Thyreoid anlage 

 Thymus anlages I * ^Trachea 



Ultimobranchial body 



FIG. 169. Diagram of the pharynx and its derivatives. (Modified after Groschuff and Kohn.) 

 I-V, first to fifth pharyngeal pouches. 



usually passing ventral to the left vena anonyma. Their upper ends 

 become attentuate and atrophy, but may persist as an accessory thymus 

 lobe (Kohn). The enlarged lower ends of the anlages form the body of 

 the gland, which is thus a paired structure (Fig. 170). At 50 mm. (CR) 

 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, after which its involution begins. This process proceeds 

 slowly in healthy individuals, rapidly in case of disease. True atrophy 

 of the parenchyma enters at about the fiftieth year. 



The ventral diverticulum of the fourth pouch is a rudimentary thymic 

 anlage. It usually atrophies. 



