DEVELOPMENT OF THE PHARYNX AND NECK. 



45 



become hypertrophoid. The lymphoid tissue of the naso-pharynx, 

 when hypertrophied, may press on and obstruct the Eustachian 

 tube and respiratory space (see Figure 33). 



The Lingual Tonsil. — That part of the tongue (pharyngeal) 

 produced between the 2nd and 3rd arches is covered by 

 mucous glands which are surrounded by nodules of lymphoid 

 tissue — the collective glandular mass receiving the name of 

 lingual tonsil. It will thus be seen that from the 2nd cleft is 

 produced a circum-pharyngeal ring of lymphoid tissue of great 

 physiological and pathological importance. 



The Thymus. — The thymus arises in the same manner as the 

 tonsil, only from the 3rd instead of the 2nd cleft (Figs 33 

 and 34). The 3rd cleft is represented in the adult by the 



tuberculum impar. 



1st recess (salivary glands) 

 2nd recess (tonsil) 

 median thyroid bud. 

 -Jurcula (epiglottis) 

 3rd recess (thymus) 

 4th recess (lat. thyroid bud) 

 aryteno. epi. fold 

 coelom (pericardium) 

 pulmonary groove 



stomach 



Fig. 34. — Showing the origin of the Tonsil, Thymus, and Thyroid from the Internal 

 Cleft Recesses during the 4th week. (After His.) 



space in front, and on each side of the epiglottis. The hypoblast 

 in the 3rd cleft recess thickens, becomes pushed out as a minute 

 pouch, shaped like a Florence flask, between the 3rd (internal 

 carotid) and 4th (arch of aorta) vascular arches. The neck of 

 the glandular hypoblastic pocket disappears. By a species of 

 secondary budding it becomes broken up into islands or separated 

 acini. Either by a production of broods of lymphoid cells within 

 the hypoblastic epithelium or by an invasion of lymphoid cells. 



