THE LATER DEVELOPMENT OF THE FROG 219 



visceral pouches, then remain separate from one another as 

 the first to fourth ceratobranchials. 



The arrangement of these arches is profoundly modified 

 during metamorphosis, when the gill slits close and the jaw 

 articulation moves backward. The hyoid bar, or ceratohyal, 

 loses its connection with the palato-quadrate, and becomes 

 considerably reduced in diameter. The copula becomes re- 

 duced and a pair of new cartilages develops each side of it, 

 connecting with the hypobranchial plate and hyoid elements; 

 these are the manubrial cartilages (Fig. 84, A). 



The hyobranchial apparatus of the fully metamorphosed frog 

 consists of a broad median plate of cartilage formed of the fused 

 manubria, copula, and hyobranchial plate (Fig. 84, B). The 

 ceratohyals remain as slender processes of this, known as the 

 hyoid cornua. Other processes of the median cartilage develop 

 anew, and only the posterior processes formed from the 

 second branchials, represent primary elements of the branchial 

 series; the other elements disappear entirely. 



C. THE DERMAL ELEMENTS 



The derm bones of the skull begin to appear in the dermal 

 layer of the integument covering the head and lining the mouth 

 before any other bony elements of the skeleton are indicated. 



The first derm bone to appear is the median parasphenoid, in 

 the roof of the mouth of the tadpole of about 20 mm. (shortly 

 after the hind-legs appear). This bone finally becomes dagger- 

 shaped and covers the large basicranial fontanelle of the chon- 

 drocranium. The paired f rentals and parietals appear some- 

 what later, roofing the cranium and covering the supracranial 

 fontanelle; these elements later unite forming the paired fronto- 

 parietals (Fig. 82, B). A pair of nasals roof the olfactory 

 capsules, while within the capsules appear the septo-nasals 

 (intranasals) . 



During metamorphosis the dermal elements of the mandibu- 

 lar arch and the other bones of the mouth appear. Premaxillce 

 and maxillce form the margin of the upper jaw, connecting later 



