. The development of the chick; an introduction to embryology. Birds -- Embryology. FROM TWELVE TO THIRTY-SIX SOMITES 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 mandibular in front of the first pouch, form- ing als
. The development of the chick; an introduction to embryology. Birds -- Embryology. FROM TWELVE TO THIRTY-SIX SOMITES 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 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, (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 vPl yC.(]J ^ ^,. 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. Understanding of their relations is therefore essential to knowledge of the develoj^ment 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. Please note that these images are extracted from scanned page imag
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