174 THE DEVELOPMENT OF THE CHICK 



bryologically first by the development of structures, the visceral 

 arches and clefts, whose primary function was aquatic respira- 

 tion, and second by the development of the air-breathing 

 lungs. Such fundamental changes in function have left a deep 

 impression, not only on the embryonic history of the pharynx 

 itself, but also on the development of the nervous and vascular 

 systems. 



The extreme anterior end of the pharynx extends at first 

 some distance in front of the oral plate, and may hence be called 

 the pre-oral gut (Figs. 75, 85, etc.). After the rupture of the 

 oral plate, the pre-oral gut appears like an evagination of the 

 pharynx immediately behind the hypophysis and is now known 

 as Seessel's pocket (Fig. 87), but it gradually flattens out and 

 disappears (Fig. 88). 



The form of the pharynx at thirty-three hours has been 

 already described; briefly, it is much expanded laterally , exhibiting 

 a crescentic form in cross-section (Fig. 54 A). The horns of the 

 crescent are in contact with the ectoderm in front of the auditory 

 pit, marking the site of the future hyomandibular cleft, which 

 arises by perforation in the fused area at about forty-six hours. 

 A second pair of lateral expansions brings about a second fusion 

 of the lateral wings of the pharynx just behind the auditory pit 

 at about the stage of 19-20 somites. This is .followed by the 

 formation of a third and a fourth pair of lateral evaginations of 

 the pharynx which reach the ectoderm at about 23 s and 35 s 

 respectively. The walls of the pharynx appear considerably 

 constricted between the evaginations which are known as vis- 

 ceral pouches (Figs. 100 and 101). 



Corresponding to each visceral pouch there is formed an 

 ectodermal invagination of much lesser extent, which may be 

 known as the visceral furrow. The furrows do not form directly 

 opposite the pouches, but slightly behind them so as to overlap 

 the margins of the latter (Fig. 101). The ectoderm of the visceral 

 furrows forms a close union with the entoderm of the pouches, 

 and openings arise within these areas, excepting the fourth, 

 forming transitory visceral clefts. 



There are thus four pairs of visceral pouches and furrows, 

 known as the first, second, third, and fourth; the first is some- 

 times called the hyomandibular. 



According to Kastschenko, there are evidences of three pairs of 



