176 THE DEVELOPMENT OF THE CHICK 
an intermediate free portion. At about the 26s stage an opening 
(cleft) forms at the dorsal adhesion, but none at the ventral; 
thus the first visceral cleft is confined to the dorsalmost portion 
of the pouch (Fig. 100). This opening closes about the end of 
the fourth day; the ventral portion of the pouch then flattens 
out, and the dorsal portion expands upwards towards the otocyst 
(Fig. 102). 
The first visceral (mandibular) arch thickens greatly between 
the 14 and 35s stages, the ventral ends project a little behind 
the oral invagination, and subsequently meet to form the primor- 
dium of the lower jaw (Figs. 125 and 126, Chap. VII). A pro- 
ules VP. P, 
A ee 
UAL | UPF 
28) IAATCT. of, 
} 
Fic. 101. — Frontal section through the pharynx of a 35s embryo. 
a.a. 1, 2, 3, 4, First, second, third, and fourth aortic arches. Hyp., Hypo- 
physis. J., Jugular vein. lar-tr. Gr., Laryngotracheal groove. or., Oral 
cavity. Ph., Pharynx. Th., Thyroid. v. A. 1, 2, 3, First, second, and third 
visceral arches. v. C. 1, First visceral cleft. v. F. 2, 3, Second and third 
visceral furrows. v. P. 2, 3, 4, Second, third, and fourth visceral pouches. 
IH, Third cranial nerve. 
jection of the upper anterior border just behind the eye is 
the beginning of the maxillary process, or primordium of the 
maxillary portion of the upper Jaw. 
The second visceral pouch likewise becomes adherent to the 
ectoderm of the second visceral furrow at its dorsal and ventral 
ends, and openings are formed in each adhesion by the 35s stage 
(Fig. 190); the dorsal opening is small and oval (later becoming 
more elongated) while the ventral one is a long, narrow fissure; 
they are separated only by a narrow bridge of tissue, and close 
during the fourth day. 
The third visceral pouch behaves like the second, forming a 
