44 HUMAN EMBRYOLOGY AND MORPHOLOGY. 



The Tonsil. — The tonsil arises comparatively late in foetal 

 life. In the 4th month eight or ten isolated buds of hypoblast 

 push out from the second cleft (Fig. 34) into the mesoblastic 

 tissue in the wall of the pharynx above the basal part of the 

 tongue. The buds form the crypts and glandular tissue of the 

 tonsil. Lymphoid tissue — for the tonsil must be regarded as a 

 lymphoid structure — collects round these glandular buds. 



Concerning the origin of the lymphoid cells, both of the tonsil 

 and the thymus, there are two quite distinct theories. The more 

 recent (Gulland's) is that the epithelial (hypoblastic) cells, which 

 form the glandular buds of the tonsil, give rise to broods of 

 lymphoid cells ; the older, that these lymphoid cells arise from 

 the blood or surrounding connective tissue, creep in and form 

 follicles round the glandular hypoblastic buds. The tonsil rests 

 on the superior constrictor and the pharyngeal fascia, or inner 

 sheath of the constrictor muscles, which surrounds it and forms 

 its capsule. 



Over the tonsil and between the pillars of the fauces is the 

 supra-tonsillar recess (Fig. 33), a remnant of the second cleft. 

 The Plica triangularis is a fold of mucous membrane which is 

 continued from the anterior pillar of the fauces to the under 

 surface of the soft palate, overhanging the supra-tonsillar recess 

 (Fig. 33). It is well marked in the foetus, but commonly dis- 

 appears before adult life. 



The tonsil is part of a great lymphoid system stationed along 

 the alimentary canal. It reaches its fullest growth in youth, as 

 is the case with the lymphoid system generally ; when active 

 growth of the system is over, and especially in the years of decay, 

 it becomes markedly reduced in size. 



The Pharyngeal Recess and Pharyngeal Tonsil.— At each 

 side, the roof of the pharynx is produced outwards, behind the 

 Eustachian tube and levator muscles of the palate, to form the 

 lateral recesses of the pharynx. They represent the upper ends 

 of the second cleft, the palate (from the maxillary processes) 

 having grown backwards inside the first and second arches and 

 separated the tonsillar part of the second cleft from the pharyngeal 

 recess. In the recess, and especially on the posterior wall of the 

 pharynx between the recesses and round Seessel's pocket, there is 

 developed much lymphoid tissue, the pharyngeal tonsil, which may 



