DEVELOPMENT OF THE ALIMENTARY TUBE AND APPENDED ORGANS. 331 



Since the branchial arches develop in such a way that they are successively 

 smaller from the first to the fourth, the pharyngeal cavity becomes funnel- 

 shaped (Fig. 294). It also becomes somewhat flattened in the dorso- ventral 

 direction, and in the earlier stages when the arches and grooves are fully formed, 

 the pharynx constitutes approximately one-third the entire gut (Fig. 285). 

 Primarily the pharyngeal cavity is separated from the oral cavity by the pharyn- 

 geal membrane (see p. 319; also Fig. 282). When this ruptures and disappears 

 (during the fourth week ?) the two cavities are in open communication. What 

 point in the adult represents the attachment of the pharyngeal membrane is 

 not known; but the glosso- and pharyngopalatine arches (pillars of the fauces) 

 are usually considered as the boundary between the mouth and pharynx. The 

 caudal limit of the pharynx is the opening of the larynx (Figs. 285 and 294). 



Thus in the early stages the general adult character of the pharynx is es- 

 tablished. While the branchial arches and grooves undergo profound changes, 

 the pharyngeal cavity retains the same relation to the mouth and to the oeso- 

 phagus and respiratory tract. The cavity becomes relatively shorter, however, 

 and the alternating ridges and pockets in its walls are lost as the arches and 

 grooves are transformed into other structures. The metamorphosis of the 

 arches and grooves is considered elsewhere (p. 149). 



THE TONSILS. The tonsils arise in the region of the ventral part of the 

 second inner branchial groove. During the third month the epithelium 

 (entoderm) grows into the underlying connective (mesenchymal) tissue in the 

 form of a hollow bud. From this, secondary buds develop, which are at first 

 solid, but later (during the fourth or fifth month) become hollow by a disappear- 

 ance of the central cells and open into the cavity of the primary bud, thus form- 

 ing the crypts. Lymphoid cells wander from the neighboring blood vessels, or 

 are derived directly from the epithelium (Retterer), and with the connective 

 tissue form a diffuse lymphatic tissue under the epithelium (Fig. 295). By the 

 eighth month the cells become more numerous in places, and by the third 

 month after birth form distinct lymph follicles with germinal centers. The 

 formation of follicles goes on slowly and is probably not complete until 

 some time after birth. 



The Lingual Tonsils. The lymphatic tissue of the tongue develops in rela- 

 tion to the lingual glands. During the eighth month lymphoid infiltration 

 occurs around the ducts of the glands, and the connective tissue acquires the 

 reticular character. True follicles probably do not appear until the child is at 

 least five years old. 



The Pharyngeal Tonsils. During the sixth month small folds appear in the 

 mucous membrane of the roof of the pharynx and become diffusely infiltrated 

 with lymphoid cells. This occurs first in the posterior part of the roof, but later 

 (seventh or eighth month) it extends forward and along the sides of the naso- 



