GILL CLEFTS. 167 



epithelium may become that of the tonsil. The upper portion of the de- 

 pression made by the second pouch probably becomes the pharyngeal 

 recess [fossa of Rosenmiillerj. The third pouch, near where it meets the 

 ectoderm, sends a tubular diverticulum (th) down the neck behind the 

 thyreoid gland; it continues into the thorax, lying ventral to the arch of the 

 aorta (Fig. 189). The diverticulum loses its lumen and becomes detached 

 from the pharynx; it forms the thymus. Besides this elongated structure, 

 the third pouch produces a rounded clump of cells which becomes sepa- 

 rated from the upper or anterior end of the thymus. This nodulus thymi- 

 cus has been said to produce the glomus caroticum; but the latter is now 

 generally regarded as a vascular mesenchymal 

 structure. The nodulus thymicus has also been said 

 to form a small body attached to the posterior sur- 

 face of the thyreoid gland in the adult, and called 

 the paraihyreoid gland. The origin of the parathy- 

 reoid glands, of which there may be four in man, two 

 on either side, is still uncertain; and the fate of 

 the nodulus thymicus is obscure. The fourth 

 pharyngeal pouch (4) soon becomes Y-shaped by F IG - s 9 . 



... i i i i / 7 \ rr-ii i The thymus, th., and thyreoid , 



union with the postbranchial body (p.b.). The latter t., of 29 mm. human em- 

 bryo; p., para thyreoid 



is an independent outgrowth of the pharynx, ans- sffnd [derived from the 



r J 3d pouch (?)]; p. g., 



ing near the fourth pouch, and considered either a nv^d^from'the^h^oLlfh 

 rudimentary fifth pouch, or a structure not related jjgj {g e p t y h r yS| 

 to the pouches. It elongates and fuses with the thy- %&?'' '"(After* v 

 reoid gland, from the tissue of which it is scarcely 



to be distinguished. Embryologists differ as to whether it forms any of 

 the adult thyreoid gland. The fourth pouch itself produces a nodule of 

 tissue which has been said to form the anterior pair of parathyreoid glands, 

 but its fate is still uncertain. 



Since the derivatives of the first pouch are to be described with the ear, 

 it remains to consider the palatine tonsils, as related with the second pouch; 

 the thymus, as derived from the third; the thyreoid, from the floor of the 

 mouth and from the postbranchial bodies; and the parathyreoid glands 

 from the third ( ?) and fourth ( ?) pouches. 



PALATINE TONSILS. 



The palatine tonsils are two rounded masses of lymphoid tissue, one 

 on either side of the throat, between the arches of the palate. They are 

 covered by the mucous membrane or tunica mucosa, which throughout the 

 digestive tract consists of several layers. The entodermal epithelium rests 

 on a connective or reticular tissue layer, the tunica propria. A structure- 



