164 THE ENTODERMAL CANAL AND THE BODY CAVITIES 



THE ULTIMOBRANCHIAL OR POSTBRANCHIAL BODIES 



The ultimobranchial body is the derivative of the fifth pharyngeal pouch 

 (Fig. 169). With the atrophy of the duct of the fourth pouch it is set free and 

 migrates caudad with the parathyreoids. It forms a hollow vesicle which has 

 been erroneously termed the lateral thyreoid. According to Grosser (Keibel and 

 Mall, vol. 2) and Verdun, it takes no part in forming thyreoid tissue, but atrophies. 

 Kingsbury (Anat. Anz., Bd. 47, 1915) denies the origin of the ultimobranchial 

 body from any specific pouch, and asserts it is "merely formed by a continued 

 growth activity in the branchial entoderm." 



THE THYREOID GLAND 



In embryos with five to six primitive segments (1.4 mm.) there appears in 

 the mid- ventral wall of the pharynx, between the first and second branchial arches, 

 a small out-pocketing, the thyreoid anlage. In 2.5 mm. embryos it has become a 

 stalked vesicle (Figs. 167 B and 87) Its stalk, the thyreoglossal duct, opens at the 

 aboral border of the tuberculum impar of the tongue (Fig. 157 ^); this spot is 

 represented permanently by the foramen ccecum (Fig. 180). The duct soon 

 atrophies and the bilobed gland anlage (Fig. 169) loses its lumen and breaks up 

 into irregular solid anastomosing plates of tissue as it migrates caudad. It takes 

 up a transverse position with a lobe on each side of the trachea and larynx (Fig. 

 170). In embryos of 24 mm. discontinuous lumina begin to appear in swoUen 

 portions of the plates; these represent the primitive thyreoid follicles (Norris, 

 Amer. Jour. Anat., vol. 20, 1916). 



LARYNX, TRACHEA AND LUNGS 



In embryos of 23 segments, the anlage of the respiratory organs appears as a 

 groove in the floor of the entodermal tube just caudal to the pharyngeal pouches. 

 This groove produces an external ridge on the ventral wall of the tube, a ridge 

 which becomes larger and rounded at its caudal end (Fig. 171). The laryngo- 

 tracheal groove and the ridge are the anlages of the larynx and trachea. The 

 rounded end of the ridge is the unpaired anlage of the lungs. 



Externally two lateral longitudinal grooves mark off the dorsal esophagus 

 from the ventral respiratory anlages. The lung anlage rapidly increases in size 

 and becomes bilobed in embryos of 4 to 5 mm. A fusion of the lateral furrows 

 progressing cephalad, constricts first the lung anlages and then the trachea from 

 the esophagus. At the same time the laryngeal portion of the groove and ridge 

 advances cranially until it hes between the fourth branchial arches (Fig. 87). At 



