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10-11-2011
[block 1] The Branchial or Visceral Arches and Pharyngeal Pouches. -In the lateral walls of the anterior part of the fore-gut five pharyngeal pouches appear (Fig. 42). $newpage [block 1] The Branchial or Visceral Arches and Pharyngeal Pouches. -In the lateral walls of the anterior part of the fore-gut five pharyngeal pouches appear (Fig. 42). $newpage [block 2] Each of the upper four pouches is prolonged into a dorsal and a ventral diverticulum. Over these pouches corresponding indentations of the ectoderm occur, forming what are known as the branchial or outer pharyngeal grooves. $newpage [block 2] Each of the upper four pouches is prolonged into a dorsal and a ventral diverticulum. Over these pouches corresponding indentations of the ectoderm occur, forming what are known as the branchial or outer pharyngeal grooves. $newpage [block 3] The intervening mesoderm is pressed aside and the ectoderm comes for a time into contact with the entodermal lining of the fore-gut, and the two layers unite along the floors of the grooves to form thin closing membranes between the fore-gut and the exterior. Later the mesoderm again penetrates between the entoderm and the ectoderm. In gill-bearing animals the closing membranes disappear, and the grooves become complete clefts, the gill-clefts, opening from the pharynx on to the exterior; perforation, however, does not occur in birds or mammals. $newpage [block 3] The intervening mesoderm is pressed aside and the ectoderm comes for a time into contact with the entodermal lining of the fore-gut, and the two layers unite along the floors of the grooves to form thin closing membranes between the fore-gut and the exterior. Later the mesoderm again penetrates between the entoderm and the ectoderm. In gill-bearing animals the closing membranes disappear, and the grooves become complete clefts, the gill-clefts, opening from the pharynx on to the exterior; perforation, however, does not occur in birds or mammals. $newpage [block 4] The grooves separate a series of rounded bars or arches, the branchial or visceral arches, in which thickening of the mesoderm takes place (Figs. 40 and 41). The dorsal ends of these arches are attached to the sides of the head, while the ventral extremities ultimately meet in the middle line of the neck. $newpage [block 4] The grooves separate a series of rounded bars or arches, the branchial or visceral arches, in which thickening of the mesoderm takes place (Figs. 40 and 41). The dorsal ends of these arches are attached to the sides of the head, while the ventral extremities ultimately meet in the middle line of the neck. $newpage [block 5] In all, six arches make their appearance, but of these only the first four are visible externally. The first arch is named the mandibular, and the second the hyoid; the others have no distinctive names. In each arch a cartilaginous bar, consisting of right and left halves, is developed, and with each of these there is one of the primitive aortic arches. $newpage [block 5] In all, six arches make their appearance, but of these only the first four are visible externally. The first arch is named the mandibular, and the second the hyoid; the others have no distinctive names. In each arch a cartilaginous bar, consisting of right and left halves, is developed, and with each of these there is one of the primitive aortic arches.
[block 1] The Branchial or Visceral Arches and Pharyngeal Pouches. -In the lateral walls of the anterior part of the fore-gut five pharyngeal pouches appear (Fig. 42). [block 2] Each of the upper four pouches is prolonged into a dorsal and a ventral diverticulum. Over these pouches corresponding indentations of the ectoderm occur, forming what are known as the branchial or outer pharyngeal grooves. [block 3] The intervening mesoderm is pressed aside and the ectoderm comes for a time into contact with the entodermal lining of the fore-gut, and the two layers unite along the floors of the grooves to form thin closing membranes between the fore-gut and the exterior. Later the mesoderm again penetrates between the entoderm and the ectoderm. In gill-bearing animals the closing membranes disappear, and the grooves become complete clefts, the gill-clefts, opening from the pharynx on to the exterior; perforation, however, does not occur in birds or mammals. [block 4] The grooves separate a series of rounded bars or arches, the branchial or visceral arches, in which thickening of the mesoderm takes place (Figs. 40 and 41). The dorsal ends of these arches are attached to the sides of the head, while the ventral extremities ultimately meet in the middle line of the neck. [block 5] In all, six arches make their appearance, but of these only the first four are visible externally. The first arch is named the mandibular, and the second the hyoid; the others have no distinctive names. In each arch a cartilaginous bar, consisting of right and left halves, is developed, and with each of these there is one of the primitive aortic arches.
10-11-2011
10-11-2011
10-11-2011
10-11-2011
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