200 



THE ENDODERMAL DERIVATIVES 



base of the second pair of visceral arches, just before the time of 

 hatching and at about the 5 mm. stage. It later becomes a bi-lobed 

 and solid organ, far removed from its site of origin. Only its original 

 lining is endodermal, the bulk of the gland being mesodermal in 

 origin. At about the 10 mm. stage, however, it becomes separated 

 from the pharyngeal floor as a closed sac. This divides into two lobular 

 and vesicular masses, and moves to a position on either side of 

 the hyoid cartilage apparatus. The thyroid gland of the 15 mm. 

 stage is divided but retains a connection at its anterior ends by a 

 short isthmus, so that it takes the shape of an inverted "Y," with a 

 short base. The wings move posteriorly, along the ventral face of 

 the hyoid cartilage, and these changes are correlated with gradual 

 changes in the hyoid apparatus. The glands enlarge, the surround- 



Thyroid 

 Sucker Pharynx evagmation Hyoid arch Pharynx Thyroid 



Thyroid 

 Hyoid 



cartilage 



Notochord 



Infundibulum 



Hypophysis 



Heart 



Hyoid Thyroid 

 cartilage gland 



Tongue 

 muscles 



Early development of the thyroid gland of the frog. (A,B) Separation of the 

 thyroid anlage from the pharynx. (C) Division of the thyroid anlage into lobes 

 at the 9 mm. stage. (D) Sagittal section of the 11 mm. stage showing the 

 position of the thyroid anlage in relation to the pharynx. 



(Continued on facing page.) 



