174 THE DEVELOPMENT OF THE CHICK 
bryologically first by the development of structures, the viscera! 
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 (lig. 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 23s and 35s 
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 cle/ts. 
There are thus four pairs of visceral pouches and furrows, 
known as the first, second, third, and fourth; the first 1s some- 
times called the hyomandibular. 
According to Kastschenko, there are evidences of three pairs of 
