THE PHARYNX AND THYROID BODY. 153 



a short median longitudinal groove appears along the floor of the 

 pharynx (Fig. 69, th). The groove is shallow anteriorly, but 

 deepens at its hinder end, where it leads into a small, conical, pit- 

 like depression of the hypoblast forming the pharyngeal floor, 

 just in front of the pericardial cavity (Fig. 69, CP.) 



At a later stage, shortly before the opening of the mouth, 

 the median groove is still present. The pit at its hinder end has 

 deepened slightly, and the hypoblast cells, forming the floor of 

 the pit, have grown back as a solid rod of cells (Fig. 64, th), 

 closely connected at its hinder end with the anterior wall of the 

 pericardium ; this solid rod of cells becomes the thyroid body. 



Soon after the mouth opens, the thyroid body separates com- 

 pletely from the floor of the pharynx, remaining as a solid 

 rounded mass of pigmented cells, in close contact with the 

 anterior wall of the pericardium. A little later, in tadpoles of 

 about 12 mm. length (Fig. 65, th), the thyroid body becomes 

 divided into right and left halves by the growth downwards of 

 a median keel from the basihyal cartilage (Fig. 65, hb). The 

 two halves remain connected by a narrow bridge of cells below 

 the cartilage for a short time, but soon separate and become the 

 paired thyroid bodies of the adult frog. 



After their separation the thyroid bodies increase consider- 

 ably in size ; they are at first solid, but the component cells soon 

 become arranged in strings, which become hollowed out along 

 their axes, and so form a series of rounded or oval vesicles, which 

 communicate freely with one another, and are filled with fluid. 



The thyroid bodies are very vascular ; they lie in the floor of 

 the mouth, a short way in front of the glottis, immediately to the 

 inner sides of the lingual arteries, which supply them, and along 

 the course of the lingual veins. 



6. The (Esophagus. 



The oesophagus is formed from the most anterior part of the 

 narrow or intestinal region of the mesenteron, and leads directly 

 from the pharynx. It is at first tubular, but in tadpoles of 

 about 8 mm. length, shortly before the opening of the mouth, the 

 cavity of the oesophagus becomes completely blocked up, by pro- 

 liferation of the cells forming its walls (Fig. 64, to). This 

 solid portion of the oesophagus lies immediately behind the 

 pharynx, and has a length of about 0-15 mm. The solid con- 



