204 GENERAL BIOLOGY 



The fate of the gill clefts and skeletal arches is no less 

 interesting. The foremost cleft narrows to a tubular pas- 

 sage way, in connection with the outer end of which the ear 

 is developed, and the cleft in part persists as the eustachian 

 tube leading from the pharynx to the middle ear; this is its 

 fate in most higher vertebrates, and in ourselves. The 

 other clefts close after the atrophy of the gills. The skeletal 

 arches that surround the neck, standing between the clefts 

 have various fates. The mandibular arch that stands in the 

 wall before the first cleft in the salamander, becomes the 

 mandible, or lower jaw of the adult salamander (note that 

 the tadpole has no mandible, and the form of its mouth is 

 entirely changed in metamorphosis). The succeeding 

 skeletal arches of the larval salamander persist in part as the 

 paired members of the hyoid skeleton shown in figure 112; 

 the foremost of these persists as the single hyoid bone in 

 most of the higher vertebrates and in ourselves. 



The kidney. — The development of the kidney is no less 

 remarkable. In that part of the cleft mesoderm, which 

 enters into the composition of the body wall (the somatic 

 layer) a cleft appears which develops into a long tube or 

 duct, (fig. 122 c, n) that extends from a point on the ven- 

 tral wall of the archenteron, where the urinary bladder will 

 later develop, forward along the wall of the coelom. At its 

 anterior end several short convolute tubes develop, with 

 their open ends abutting against a dilatation of the dorsal 

 aorta in the coelom (fig. 131a). Thus, they are probably 

 able from the beginning, by reason of this connection, to 

 remove nitrogen waste from the blood. These tubules in- 

 crease in number, length and complexity of arrangement 

 and form the pronephros, or "head kidney." 



The true kidney develops later, and much farther back in 

 the coelom. It begins as separate and isolated tubules 

 developing against the dorsal wall along side the pronephric 



