50 Vertebrate Embryology 



or oesophagus, and a ventral portion or larynx. 

 From the laryngeal chamber the lungs arise 

 as hollow lateral outgrowths, some time after 

 hatching, when the tadpole is about 8 mm. 

 long. At about the time of the formation of the 

 lungs the tubular oesophagus becomes solid 

 and remains closed until after the formation 

 of the oral opening. What the significance of 

 this curious fact may be is not known. 



The thyroid body begins, at about the time 

 of hatching, as a small, median depression in the 

 floor of the pharynx. The depression becomes 

 deeper, especially at its posterior end, and 

 finally loses its connection with the pharynx 

 and lies as a solid rod of cells just in front of 

 the pericardium. When the tadpole is about 

 12 mm. in length the thyroid becomes sepa- 

 rated into right and left halves by the growth 

 of a median longitudinal septum, and after 

 considerable growth each of these halves is 

 converted into the adult structure by the re- 

 arrangement of its cells to form the round or 

 oval vesicles that are characteristic of the thy- 

 roid gland. 



The bladder arises at about the time of 

 metamorphosis as an outgrowth from the ven- 

 tral wall of the hind gut. 



