206 



STRUCTURE OF THE VERTEBRATES 



first anterior pouch forming the spiracle. This structure in the 

 dogfish carries a rudimentary gill on its posterior surface. In 

 most of the higher fish the spiracle is completely lost, although 

 it is retained by the crossopterygian fish, and from them in- 

 herited by the amphibia as the Eustachian tube and middle ear. 

 The embryos of all vertebrates develop the gill pouches and, 

 with doubtful exceptions, these break through to the outside of 



GUI slit 



Nerve cord 

 Notochord 



Gill slit 



Nerve cord 

 Notochord- 

 Pharynx 

 Gill pouch 



Fig. 119. Embryology of the Gill Slits (Diagrammatic). Drawings (A), 



(B), and (C) are in horizontal section. Cross sections of similar stages are 



placed immediately below. The gill pouches begin as outpocketings of the 



pharynx and meet invaginations of the ectoderm. 



the animal as gill slits. Gills develop only in the cyclostomes, 

 fish and amphibia, and are never found in amniote embryos. The 

 septum between each gill pouch is supported by a gill bar, or 

 branchial cartilage. These branchial cartilages become variously 

 modified in the vertebrates. (See Visceral Skeleton, page 143.) 

 A horizontal section through the human embryonic pharynx 

 illustrates the typical vertebrate condition. The five paired 

 branchial evaginations extend laterally from the pharynx. With 

 the exception of the first, these pouches are never functional; 

 but from each pouch develops a proliferation of cells which be- 



