332 TEXT-BOOK OF EMBRYOLOGY. 



pharygeal cavity. By the end of foetal life the ridges become quite large. 

 Follicles may appear before birth or not until one or two years later. After 

 puberty the ridges almost completely disappear, but the adenoid tissue remains 

 wholly or in part. 



The bursa pharyngea is an evagination from the roof of the pharynx about 

 the upper border of the superior constrictor muscle, and is apparent in em- 

 bryos of eleven weeks. It probably has no genetic relation to the hypophysis. 

 Its significance is not known. 



FIG. 295. Section through the middle of the developing tonsil of a human 



embryo of 5 months. Stohr. 



6, Epithelial buds (secondary outgrowths) from the epithelium lining the primary crypt (c) ; 

 L, lymphoid infiltration of the connective (mesodermal) tissue. 



THE BRANCHIAL EPITHELIAL BODIES. 



THE THYREOID GLAND. The thyreoid arises, after the manner of ordinary 

 glands, as an evagination from the epithelium of the pharynx. It appears in 

 embryos of 3 to 5 mm. as a ventral outgrowth of epithelium in the floor of the 

 pharynx, at the point where the tuberculum impar and the two paired anlagen 

 of the tongue join (Fig. 296). This point is the foramen caecum linguae which 

 has already been mentioned in connection with the development of the tongue 

 (p. 322). The evagination grows into the mesodermal tissue in the ventral wall 

 of the neck, and forms a transverse mass of epithelium. The latter breaks up 

 into irregular cords of cells which, by a further process of budding, grow cau- 

 dally along the ventral surface of the larynx. The cords of cells are from the 

 first surrounded by connective tissue and later also become surrounded by net- 

 works of capillaries (Fig. 297). They ultimately break up into smaller masses 

 which become hollow and form the alveoli. Colloid secretion begins toward 

 the end of foetal life or soon after birth. 



As the gland grows toward its final position it becomes enlarged laterally into 

 the two lateral lobes, which remain connected by the isthmus (Fig. 298). The 

 pyramidal process represents either a secondary outgrowth from the isthmus or 

 one of the lobes, or a remnant of the original connection with the tongue, that is, 



