The Development of the Frog 77 



The exoccipitals, the frontals and parietah, which are 

 the first to separate, and other bones soon follow; and 

 by the time the metamorphosis is complete and the 

 tail absorbed, all the bones of the adult cranium are 

 present, except the sphenethmoid, which does not appear 

 till some months later. 



" b. The sense capsules. The cartilaginous auditory 

 capsules appear in tadpoles of about 12 mm. length as 

 thin shells of cartilage investing the auditory vesicles. 

 They are at first quite independent of the cranium, but 

 before the completion of the opercular folds they fuse 

 with the upgrowing parachordals to form part of the 

 side walls of the skull. The pro-otic appears at about 

 the time of completion of the metamorphosis. 



" The optic capsules are thin shells of cartilage, form- 

 ing part of the sclerotic coats of the eyes. They arise 

 about the same time as the auditory capsules, and, un- 

 like the other sense capsules, they remain distinct from 

 the cranium throughout life, in order to secure mobility 

 of the eyeballs. 



" The olfactory capsules are from their first appearance 

 very closely connected with the anterior ends of the 

 trabeculse, which grow up between them to form the 

 median vertical internasal septum. They develop later 

 than the auditory and optic capsules." l 



c. The visceral skeleton. The cartilaginous bars 

 lying in the visceral arches make up what is known 

 as the visceral skeleton, and as the structure and fate 

 of these bars were described in a previous section, 

 a more detailed discussion will be left until the similar 

 structures in the chick are taken up. 

 1 Marshall. 



