DEVELOPMENT OF THE DIGESTIVE TUBE 603 



2. Histogenesis and Morphogenesis of Special Areas 



a. Oral Cavity 



1) General Characteristics of the Stomodaeal Invagination. The oral cavity 

 arises as a simple stomodaeal invagination in most vertebrates. However, in 

 the toadfish, Opsanus (Batrachus) tan, two stomodaeal invaginations occur 

 which later fuse to give origin to a single oral cavity (Piatt, 1891). In 

 Amphioxus, the mouth originates on the left side of the head as shown in figure 

 249D and F; later, it migrates ventrally to a median position. In cyclostomes, 

 the original invagination becomes partly everted secondarily, so that the pi- 

 tuitary invagination eventually lies on the upper portion of the head (fig. 

 283A, B). 



2) Rudiments of the Jaws. In the shark embryo, the mandibular visceral 

 arches bend to form U-shaped structures on either side of the forming oral 

 cavity and thus give origin to the primitive framework of the upper and lower 

 jaws (fig. 253). This condition holds true for other lower vertebrates, includ- 

 ing the Amphibia. In the chick, the mandibular arch bends similarly to that 

 in the shark embryo, but only the proximal portion of the upper jaw is present. 

 The anterior or distal portion is displaced by mesenchyme from the head area 

 (fig. 240). The latter condition is true also of the mammals (fig. 261). Re- 

 gardless of whether or not all the jaw framework on either side of the forming 

 oral cavity is derived from the original mandibular arch, the fact remains 

 that in the formation of the jaws, a U-shaped, mesenchymal framework on 

 either side is established in all the gnathostomous or jaw-possessing vertebrates. 



3) Development of the Tongue. The "tongue" of the shark is essentially a 

 fold of the oral membrane of the floor of the mouth, which overlies the basal 

 (hypobranchial) portion of the hyoid visceral arch. A true, flexible tongue, 

 however, is never developed in the shark or other fishes. Flexible, protrusile 

 tongues are found almost entirely in forms which inhabit the land, where they 

 are used for the acquisition and swallowing of food. The protrusile tongue, 

 therefore, is a digestive-tract structure primarily, and its use in communica- 

 tion in the human and other species is a secondary adaptation. 



The tongue generally develops from folds or growths, associated with the 

 floor of the oral cavity and anterior branchial region. These lingual growths 

 are associated with the ventral or lower jaw portions of the hyoid and man- 

 dibular visceral arches and the ventral area between these arches. However, 

 in the frog, the tongue arises from a mass of tissue at the anterior portion of 

 the floor of the mouth between the mandibular visceral arches. It is protruded 

 from the oral cavity largely by the flow of lymph into the base of the tongue. 



The tongue of the chick and other birds is developed as a fleshy, super- 

 ficially cornified structure, overlying the anterior portion of the greatly modi- 

 fied hyoid apparatus. It arises from the tuberculum impar, a swelling located 

 in the floor of the pharyngeal area between the first and second visceral arches. 



