FROM TWELVE TO THIRTY-SIX SOMITES 175 
visceral furrows in front of the first at the 14-16s stage. These he in- 
terprets as phyletic rudiments. It is certain that the lower vertebrates 
had pouches posterior to the fourth. The post-branchial bodies (see 
p. 309) are probably rudiments of a fifth pair of pouches. 
The tissue between the visceral pouches thickens, by acecumu- 
lation of mesenchyme, to form the visceral arches, of which there 
are five, viz.: (1) the mandibular in front of the first pouch, form- 
ing also the posterior boundary of the oral cavity, (2) the hyoid 
between the first and second pouches, (8) the third visceral arch 
between the second and third pouches, (4) the jfowrth viscerat 
arch between the third and fourth pouches, and (5) the fijth 
vy P/ vo~0/ vP2 yC.aé 
aN 7 me v rte VPS 
3 ~~ y ye ae 
TOO ae Ries / ~ ho a 
Nes / > 
he —? “ ff 
“C— ae lg AS 1 
\ 3/74 aN \ \ 
\ / | Ome \ 0es 
v.C. ve \ ay NE x 
cee we ‘ va J \ 
ve ML af Stom. 
LG. 
Fig. 100. — Reconstruction of the fore-gut of a chick of 72 hours. 
(After Kastschenko.) 
Hyp., Hypophysis. lar-tr. Gr., Laryngotracheal groove.  Lg., 
Lung. Md.a., Mandibular arch. Oeces., Oesophagus.  pr’o. G., Preoral 
gut. Stom., Stomach. Th., Thyroid. v.C.d, 1, 2, Dorsal division of 
the first and second visceral clefts. v.C. v. 2, Ventral division of the 
second visceral cleft. v. P. 1, 2, 3, 4, First, second, third, and fourth 
visceral pouches. 
visceral arch behind the fourth pouch. Each arch is bounded 
internally by entoderm, externally by ectoderm. The main portion 
of its substance is formed of mesenchyme; each contains also a 
branch of the ventral aorta (aortic arch) and a branch of a cranial 
nerve. Understanding of their relations is therefore essential to 
knowledge of the development of the nervous system, vascular 
system, and skull. 
We shall now consider the history of each visceral pouch 
and arch separately: 
The first visceral pouch becomes adherent to the ectoderm 
of the first visceral furrow at its dorsal and ventral ends, leaving 
