194 THE ENDODERMAL DERIVATIVES 



"jaw," consisting of a hardened ridge, develops at the base of both 

 the dorsal and the ventral lips. 



By the time of metamorphosis the horny larval teeth and jaws are 

 lost and the mandibular arches give rise to the jaw elements of the 

 adult. The upper larval teeth are replaced by permanent teeth which 

 bear only superficial resemblance to the teeth of mammals. 



The tongue, which is ultimately attached anteriorly and will be free 

 posteriorly, begins to develop by the time of metamorphosis from a 

 proliferation of cells from the endodermal floor of the pharynx. The 

 bulk of the tongue will, of course, be of mesodermal origin but most of 

 its covering and glandular elements come from the endoderm, the 

 anterior portion being covered with stomodeal ectoderm. 



The Foregut 



This anterior portion of the original archenteron expands widely in 

 front of the yolk mass and consists of all of the derivatives from the 

 stomodeum to and including the pancreas and liver. The stomodeum, 

 the anterior covering of the tongue, and the roof of the mouth anterior 

 to the internal choanae are ectodermal and only the posterior cover- 

 ing of the tongue and mouth, beginning at about the level of the 

 thyroid gland, is endodermal. This foregut portion of the archenteron 

 therefore assumes a major role in the early development of the frog, 

 where it gives rise to three successive sets of respiratory organs (i.e., 

 external and internal gills, then the lungs) to most of the endocrine 

 organs, and to a good portion of the digestive tract. 



The Pharynx. 



The pharyngeal cavity is widely expanded anterior and ventral to 

 the yolk mass of the frog embryo. The earliest elongated and vertical 

 evaginations of the pharyngeal endoderm are known as the visceral 

 or branchial pouches. Those of the most anterior pair, which develop 

 between the mandibular and the hyoid arches, are known as the 

 hyomandibular pouches. The dorsal remnant of this first pair of 

 pouches gives rise to the Eustachian tube of the adult and connects 

 the pharynx with the middle ear chamber. The second and third pairs 

 of visceral pouches are known as the first and second branchial 

 pouches, because they give rise to gill clefts, and they develop between 

 the succeeding visceral arches. Eventually, six pairs of endodermal 

 evaginations develop in this manner, but those of the last (most 



