56 Vertebrate Embryology 



the exterior. The opercular folds are not com- 

 pleted until after the formation of the mouth. 



The internal gills arise quite early as a 

 double row of papillae on the first, second, and 

 third visceral arches, below the external gills, 

 and as a single row on the fourth arch. They 

 are very vascular, and when the external gills 

 begin to shrivel, they take up the function of 

 respiration. The inner borders of the gill 

 arches develop a sort of straining apparatus, 

 to prevent solid substances from passing from 

 the pharynx into the gill chamber. 



At the end of the tadpole life, as the lungs 

 begin to function, the gill chamber is filled and 

 gradually obliterated by the growth of lym- 

 phoid and epithelial tissue, the gill clefts are 

 closed by the fusion of their edges, and the 

 gills are almost entirely absorbed, small por- 

 tions persisting in the adult as the so-called 

 tonsils. 



THE DEVELOPMENT OF THE HEART AND 

 BLOOD VESSELS 



As has been stated above, the heart and 

 blood vessels, as well as the blood itself, are 

 formed from mesoblast, the chief point of in- 

 terest being the changes that take place at 



