The Development of the Frog 51 



DEVELOPMENT OF THE GILL CLEFTS AND 



FOLDS 



The gill clefts are five pairs of narrow, 

 vertical slits which connect the cavity of the 

 pharynx with the exterior. The portions of 

 the wall between the clefts, and also in front 

 of the first and behind the last clefts are the 

 gill folds or arches. The most anterior cleft is 

 known as the hyomandibular cleft, the others, 

 from before back, are the first, second, third, 

 and fourth gill clefts. The arch in front of the 

 hyomandibular cleft is called the mandibular 

 arch, the arch between the hyomandibular and 

 the first gill clefts is the hyoid arch, and the 

 other arches, like the clefts, are called the first, 

 second, third, and fourth. 



The gill clefts, or, as they are often called, 

 the visceral or branchial clefts or pouches, be- 

 gin to develop before the tadpole hatches, and 

 are best studied in horizontal sections. The 

 first three pairs of pouches begin almost simul- 

 taneously as evaginations of the entoblastic 

 wall of the pharynx, which push outward 

 towards the ectoblast. The third and fourth 

 pouches are formed in succession behind the 

 first three. All of the pouches grow outward 

 until they come in contact with the ectoblast, 



