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 wedege-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 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 
12s. At the stage of 26s this plate forms a deep, saucer-shaped 
depression, and at the 30s stage it is a well-developed sae with 
wide-open mouth which gradually closes, thus transforming the 
sac into a small spherical vesicle lying beneath the floor of the 
pharynx (Hig. 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 23s 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 with 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 35s (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 
