178 THE DEVELOPMENT OF THE CHICK 



The second visceral arch is the largest of the arches and over- 

 laps both the first and third. See Figs. 117 and 125 in place of 

 description. All of the arches are wedge-shaped, corresponding 

 to the wedge-like form of the hind-brain region. The fourth 

 arch is small and incomplete ventrally; the fifth a mere transitory 

 rudiment. The greatest development of the arches is at about 

 the end of the fourth day. 



According to Kastschenko the closure of the visceral clefts takes 

 place external to the meeting-place of the visceral furrows and clefts, 

 and in this way some of the ectoderm of the furrows remains attached 

 to the visceral pouches. 



The thyroid arises as a small, spherical evagination of the 

 epithelium of the floor of the pharynx situated between, and a 

 little in front of, the ventral ends of the second pair of visceral 

 pouches (Figs. 85, 87, 88, 101). In the 18-20 s stage, it is repre- 

 sented by a sharply defined plate of high, columnar cells in the 

 same situation, which may be recognized even at the stage of 

 12 s. At the stage of 26 s this plate forms a deep, saucer-shaped 

 depression, and at the 30 s stage it is a well-developed sac with 

 wide-open mouth which gradually closes, thus transforming the 

 sac into a small spherical vesicle lying beneath the floor of the 

 pharynx (Fig. 102). 



The Pulmonary Tract. The portion of the pharynx that 

 includes the visceral pouches may be called the branchial portion, 

 because it is homologous to the gill-bearing portion in fishes and 

 amphibia, and because the visceral pouches are phylogenetic 

 rudiments of branchial clefts. The larnyx, trachea, and lungs 

 develop from the ventral division of the postbranchial portion 

 of the pharynx. At about the 23 s stage a reconstruction shows 

 this respiratory division of the pharynx slightly constricted from 

 the broader branchial portion, enlarged on each side at its pos- 

 terior end and w^ith a ventral depression; the latter rapidly 

 deepens to form a narrow groove, the primordium of the larynx 

 and trachea, while the posterior lateral expansion begins to form 

 outgrowths, the primordia of the lungs and air-sacs. By the 

 stage of 35 s (Fig. 100) the postbranchial portion of the pharynx 

 has become narrow transversely and its ventral half is a deep 

 groove (laryngotracheal groove) leading back to the lung pri- 

 mordia. A true median sagittal section at this time shows the 



