The Development of the Frog 53 



The Eustachian tube and the tympanic cav- 

 ity develop near the hyomandibular cleft, but 

 it is doubtful if any such close relation exists 

 between those structures and the hyomandib- 

 ular cleft as exists in some other animals. 



The other visceral clefts persist for a con- 

 siderable time, but towards the end of the 

 tadpole stage they close up and disappear. 



The fate of the gill arches is of more im- 

 portance, but as it is more easily studied in 

 the chick, a very brief statement will suffice at 

 this time. The mandibular arch, as its name 

 would indicate, becomes converted into the 

 essential part of the lower jaw. The hyoid 

 arch, as its name indicates, forms the greater 

 part of the hyoid apparatus, while the other 

 four arches almost entirely disappear. Dur- 

 ing the larval period there is present in each 

 visceral arch a rod of cartilage, which is closely 

 joined to its fellow of the opposite side ven- 

 trally but is separated from it dorsally. Thus 

 there is in each pair of arches a U-shaped bar 

 of cartilage which serves to stiffen the walls of 

 the pharynx. 



The gills, of which there are two sets, the 

 external and the internal, are developed in 

 connection with the gill arches. 



