176 THE DEVELOPMENT OF THE CHICK 



an intermediate free portion. At about the 26 s 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 35 s 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- 



FIG. 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, y. P. 2, 3, 4, Second, third, and fourth visceral pouches. 



Ill, 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 35 s stage 

 (Fig. 100); 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 



