616 THE EMBRYOLOGY OF THE FROG 



as the three branchial arches, the various skeletal elements appear as 

 condensations in the mesenchyme, which soon become cartilaginous. 



First, a short rod appears in the mandibular arch, transverse to the 

 axis of the embryo. 



This divides the dorsal portion into the beginnings of the upper jaw 

 or palato-quadrate, and the ventral portion which is the beginning of 

 the lower jaw. The lower jaw element becomes subdivided into Meckel's 

 cartilage, which comes to form the true jaw, and the infra-rostral car- 

 tilage. 



The palato-quadrate has grown rapidly, as already described, and 

 then fused with the trabeculae. 



When the tadpole is about twenty-one millimeters long, the pos- 

 terior or quadrate portion of this same cartilage connects with the audi- 

 tory capsule. 



With metamorphosis, the mouth enlarges, and this pushes back 

 many of these structures, while the part of the palato-quadrate which 

 lies in the orbital region, softens and disappears to a considerable extent. 

 The anterior connection of palato-quadrate and trabeculae becomes the 

 future pterygoid and palatine regions. All these changes draw the jaw 

 to the posterior portion of the cranium from its original anterior position. 



The infra-rostral cartilages, which have fused together across the 

 midline, now fuse with the Meckelian cartilages to form the apex or 

 mental portion of the chin. The fused cartilages are now known as 

 mento-Meckelian cartilages. As these ossify, they fuse with the dentary, 

 which is really the chief membrane bone of the lower jaw. There is a 

 small median element between the infra-rostrals which also fuses with 

 them. 



The annulus tympanicus is the outgrowth from the quadrate car- 

 tilage which surrounds the tympanic membrane of the frog. It does not 

 complete its growth until long after metamorphosis. 



The hyoid arch, like the three branchial arches lying posterior 

 to it, makes its appearance as a pair of rods of dense tissue in the corre- 

 sponding visceral arches, though not at the same time as the others. 



The hyoid cartilage, also called the ceratohyaal cartilage, extends 

 dorsad and connects with the palato-quadrate immediately behind where 

 the jaw articulates. Ventrally, it unites with the hyoid cartilage of the 

 opposite side. (Fig. 354.) 



The first branchial cartilage also unites in the ventral midline, while 

 the remaining branchial arches do not unite in the midline ventrally, 

 but have their lower anterior ends unite with the one lying immediately 

 anterior to it, and, finally, they connect dorsally in a similar manner. 



The copula, which is a medial element, then appears in the ventral 



