2l8 HISTOLOGY 



auditory tube, and it has been thought to give rise to the pharyngeal 

 recess (fossa of Rosenmiiller), but according to Hammar such is not the 

 case. Instead, it produces only the sinus tonsillaris, into which a mound of 

 lymphoid tissue, the palatine tonsil, later projects (Fig. 207, tons.}. Above 

 the tonsil the supratonsillar fossa, which may readily be seen on looking 

 into the mouth, is to be regarded as a remnant of the original second 

 pouch (Hammar, Arch. f. mikr. Anat., 1903, vol. 61, pp. 404-458). 



The lingual and pharyngeal tonsils, which are similar in structure to the palatine 

 tonsils, develop as median structures with no relation to the pharyngeal pouches. 

 Therefore the second pouches are to be regarded as the site rather than the source of 

 the palatine tonsils; there are no tonsils in the second pouches of the rat (Hammar). 



The third pouch (Fig. 206, 3) near its junction with the ectoderm, 

 sends a tubular diverticulum (tti) down the neck behind the thyreoid 

 gland; it continues into the thorax, lying ventral to 

 the arch of the aorta (as seen in front view in Fig. 

 208). This diverticulum loses its lumen, becomes 

 detached from the pharynx, and unites with its fellow 

 on the opposite side to form the thymus. Besides 

 this elongated structure, each third pouch produces 

 _th an epithelial body, or nodulus thymicus, which is a 

 ^ round clump of cells detached from the pouch at the 



FIG. 208. upper end of the thymic diverticulum. Each epithe- 



The reouu"t, f ^f ' 29 d m^." ^ body becomes attached to the posterior surface 

 pa u r?h a y io e id br ^ikn p d of the thyreoid gland, forming the inferior pair of 

 pou"h> e ; p. grparath 3 y d parathyreoid glands (Fig. 208, p.}. 



reoid gland (derived ,, , . .., ., ,_,. - N 



from the 4th pouch)-. The fourth pouch on either side (Fig. 206, 4) gives 



p. L, pyramidal lobe of . 



the thyreoid; ao. rise to an epithelial body similar to the nodulus thy- 



aorta; v., vena cava x * 



superior. (After Ver- micus. These likewise become detached as parathy- 

 reoid glands, and they constitute the superior pair 

 (Fig. 208, p. #.). Sometimes a parathyreoid gland degenerates and dis- 

 appears, and in other cases one of them may become subdivided, but 

 typically there are four in the adult. 



Behind the fourth pouch, on either side, there is a tubular prolonga- 

 tion of the pharynx variously known as the postbranchial, ultimobranch- 

 ial or telobranchial body. As the fourth pouch becomes well formed, 

 the postbranchial body is so closely associated with it that together they 

 form a Y-shaped structure, attached to the pharynx by a common stalk 

 (Fig. 206). The postbranchial bodies then grow toward one another 

 across the front of the neck, after the manner of the thymic diverticula. 

 Their ventral ends become detached and imbedded in the thyreoid gland, 

 to the substance of which they were formerly believed to contribute. 

 There is, however, no satisfactory evidence that they produce thyreoid 

 tissue, and they are generally supposed to disintegrate. 



