GILLS OR BRANCHI^ 



261 



added that some of the stegocephals in the larval stages, had gills supported by 

 gill arches, the probabilities being that these were functional. 



Fig. 281. — Cast of oropharyngeal region of pig embryo, 17 mm. long, after Fox. 

 alf, alveo-lingual fold; elm, cervical cord of thymus; dp\ dp', dorsal apex of first and 

 second phar>'ngeal pouches; dplm, dorsal plate of thymus;/, filiform appendix of second 

 pouch; mtr, median thyreoid; llr, lateral thyreoid; jw/, submeckehan fold; stt, sulcus 

 tubo-tympanicus; tm, thymus; vf, vestibular fold of mouth. 



Gills are never developed in the amniotes, but in the embryos the 

 paired visceral pouches are formed (figs. 232, 281) five in reptiles, 



Fig. 282. — Head of human embryo with pharyngeal floor removed, after Hertwig. 

 Cut surfaces Uned. Compare with fig. 247; b, lung; cs, cervical sinus; e, eye; k, hyoid 

 arch; hd, hypophysial duct (Rathke's pocket); /, lung, Ig, lacrimal duct; md, mandible; 

 n, naris; on, oronasal groove; tr, trachea. 



four in birds and mammals — in the same way as in the fish-like forms. 

 Some of these never break through to form clefts (in man only the 

 first), although their position is indicated by grooves on the outside 



