FROM TWELVE TO THIRTY-SIX SOMITES 



>3 



175 



visceral furrows in front of the first at the 14-16 s 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 accumu- 

 lation of mesenchyme, to form the visceral arches, of which there 

 are five, viz.: (1) the tnandibular 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, (3) the third visceral arch 

 between the second and third pouches, (4) the fourth visceral 

 arch between the third and fourth pouches, and (5) the fifth 



vP2 vC..a2 



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. Oes., 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. TTnderstanding 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 



