162 OUTLINES OF CHORDATE DEVELOPMENT 



elevations on the postero-external faces of the branchial arches 

 (Figs. 64, 65). Like the external gills, these are covered with 

 a thin layer of ectoderm cells which move in, covering their 

 original endodermal coat. These rudiments soon become 

 doubled on the first three branchial arches, but remain in a 

 single row on the fourth branchial arch. They enlarge rapidly 

 and form long branched processes or gill filaments, along the 

 whole border of the arch below the external gills. The fila- 

 ments become very vascular and project freely into the oper- 

 cular cavity, where they are bathed in the respiratory current 

 entering the mouth and passing out through the gill clefts and 

 opercular tube. 



The inner borders of the branchial arches become serrated 

 by the formation of a series of papillae, forming a kind of filtering 

 organ (Fig. 64, D, E). The continued widening of the dorsal 

 part of the pharynx throws the branchial portion into a ventral 

 position, and then this whole region becomes partly separated 

 from the dorsal region by the anterior and posterior folds of 

 the pharyngeal floor; these are the velar plates (Fig. 64, D, E). 



As the period of metamorphosis approaches, the opercular 

 cavity and the gill clefts become occluded by the rapid pro- 

 liferation of the cells lining these spaces, and of the gills them- 

 selves. The most of this solid mass of cells becomes completely 

 resorbed. Various structures of the young frog are derived 

 from vestiges of the gill clefts. 



The thymus body appears just before hatching, as a solic 

 internal proliferation from the epithelium of the upper side oi 

 the first branchial pouch (second visceral, or hyobranchij 

 pouch) (Fig. 56). A similar smaller proliferation from the 

 hyomandibular pouch has only a transitory existence. Th< 

 thymus rudiment enlarges slowly, separating from the wall of 

 the pouch at about 12 mm. After metamorphosis the thymus 

 bodies are seen toward the outer surface of the head, just bacl 

 of the auditory capsule and jaw articulation. 



From the dorsal ends of the other branchial clefts somewhat 

 similar bodies are formed; these become lymphoid and appear 

 to have no correspondence with the thymus rudiments. From 



