286 THE CHICK. 



opposite the first pair of branchial arches. The hinder end of 

 the groove deepens during the third day to form a pit (Fig. 114, 

 th). The walls of this pit soon join together, obliterating the 

 cavity and giving rise to a solid plug of hypoblastic epithelium 

 (Fig. 123, th). About the end of the fifth, or early part of the 

 sixth day this plug separates from the floor of the throat as a 

 solid body, composed of epithelial cells, which lies embedded in 

 the mesoblast, immediately in front of the truncus arteriosus. 



The thyroid body soon becomes bilobed, and the lobes 

 branch out as solid strings of cells, which later on become 

 tubular. A sheath of vascular connective tissue early forms 

 around the lobes, which, as development proceeds, gradually shift 

 backwards along the neck to their adult position. 



A pair of solid bodies, formed of epithelial cells, which separate 

 from the hypoblast immediately behind the third branchial 

 pouches, and take up a position at the sides of the larynx, are 

 sometimes spoken of as accessory thyroid bodies. 



The thymus arises, on each side, as a couple of epithelial buds 

 from the walls of the second and third branchial pouches. The 

 buds soon separate from the surface, and, the two buds of each 

 side fusing together, give rise to a pair of elongated rod-like 

 bodies, lying along the sides of the neck close to the carotid 

 arteries. 



The tongue is formed as an outgrowth from the floor of the 

 pharynx, opposite the hyoid and first branchial arches. It first 

 becomes conspicuous about the sixth day, and by the eighth or 

 ninth day (Fig. 116, tn) has attained a definite shape. It is 

 formed behind the boundary line between the pharynx and 

 stomatodasum, and its epithelium is therefore of hypoblastic 

 origin. 



3. The Stomatodaeum. 



The stomatodaeum, or mouth invagination, is formed by 

 pitting-in of the ventral wall of the pharynx from the exterior. 



From the time of its first formation the ventral wall of the 

 pharynx, in front of the heart, is very thin (cf. Fig. 112). On the 

 appearance of the visceral arches, as thickenings of the side walls 

 of the pharynx, this thin-walled area on its ventral surface 



