DEVELOPMENT OF THE DIGESTIVE TUBE 619 



ings (figs. 252F; 260; 262). The entodermal pouches or outpocketings are 

 called the branchial, pharyngeal, or visceral pouches, while the epidermal (ec- 

 todermal) inpushings form the visceral or branchial grooves (furrows). The 

 mesodermal columns constitute the visceral arches. 



The number of branchial pouch-groove relationships, thus established, varies 

 in different vertebrate species. In the cyclostomatous fish, Petromyzon, there 

 are seven; in Squalus acanthias, the shark, there are six. The latter number is 

 present typically in a large number of fishes. In most frogs and salamanders, 

 there are five, pouch-groove relationships with a vestigial sixth; in the chick, 

 pig, and human, there are four. (In reptiles, birds, and mammals, the fourth 

 pouch on either side may represent a fusion of two or three pouches.) The 

 number of visceral arches, of course, varies with the number of pouch-groove 

 relationships produced, the first pair of arches being formed just anterior to 

 the first pair of pouches. The first pair of arches are called the mandibular 

 visceral arches; the second pair constitute the hyoid visceral arches; and the 

 remaining pairs form the branchial arches. 



Within each visceral arch, three structures tend to differentiate: 



( 1 ) a skeletal arch, 



(2) a muscle column, associated with the skeletal arch, and 



(3) the aortal arch, a blood vessel. 



In all water-living vertebrates, including those species which spend the larval 

 period in the water, the entoderm of the branchial pouch and ectoderm of the 

 branchial groove tend to fuse intimately and perforate to form the branchial 

 or visceral clefts, with the exception of the first, pouch-groove relationship. 

 The latter is variable. In the amphibia, the first pouch does not perforate but 

 becomes associated with the developing ear. In land forms, on the other hand, 

 the pouches, as a rule, remain imperforate or weakly so. As a rule, they con- 

 tinue unperforated in mammals. The ectoderm and entoderm of the branchial- 

 pouch-groove relationships is very thin in the chick, and openings (?) may 

 appear in the more anterior pouches. (Note: The relation of these pouches to 

 respiration is discussed in the following chapter.) 



2) Pharyngeal Glands of Internal Secretion. An important developmental 

 function of the pharynx is the formation of masses of epithelial cells from vari- 

 ous parts of the entodermal wall which serve as endocrine glands. These glands 

 are the thyroid, parathyroid, thymus, and ultimobranchial bodies. The places 

 of origin of these cellular masses and their part in the formation of the endo- 

 crine system are discussed in Chapter 21. 



3) Other Respiratory Diverticula. One of the primary functions of the 

 pharyngeal area is respiration. In most water-living vertebrates, the pharyngeal 

 pouches are adapted for respiratory purposes. However, in many water-dwelling 

 species and in all land forms, a median ventral outpushing occurs which de- 



