AMPHIBIA 



201 



mesenchyme to the palatoquadrate. The palatoquadrate is 

 soon after deflected and lengthened to occupy a position further 

 back, and the columella is thereby brought into relationship 

 with the membrane which forms the outer wall of the blind 

 pouch of the hyoid cleft ; and in this region a ring-like carti- 

 lage develops as the tympanic annulus, and it supports this 

 membrane and the enclosed area of the skin which are now 

 converted into the drum of the ear. 



The other bones of the brain case are covering bones, 

 developed, that is to say, independently of the cartilage. They 



Nasal capsule 



Palato-pterygoid 

 Sphenethmoid 



Premaxitta 



Nasal 



Maxilla X 



Fronto-parietal * 



Fontanelles 



Exoccipital 



FIG. 101. Dorsal view of frog's skull. The covering bones shown on the left 

 are unshaded, the cartilage on the right is indicated by dots, and the 

 * cartilage ' bones by broken lines. 



are conveniently called membrane bones, although some of 

 these bones may be the products of both processes. The 

 membrane bones of the skull are early formed. They have to 

 be removed to disclose the cartilaginous structures which they 

 cover. This may be most readily accomplished by steeping 

 the head of the frog in glycerin. 



The roof of the skull is occupied by the triangular nasals 

 and the fused front o-parietals. The floor is protected by the 

 dagger-shaped parasphenoid and the vomers. The premaxillae 

 and the maxillae form, with a short quadrato-jugal or quadrato- 

 maxilla, the outer arcade of the roof of the mouth ; the palatines 



