THE LATER DEVELOPMENT OF THE FROG 215 



The ethmoid region remains comparatively simple throughout 

 the tadpole stage; its extreme complication comes later. 

 During the larval period the internasal septum extends dorsally, 

 forming the anterior wall of the cranial cavity, perforated by 

 the olfactory nerves. The trabecular cornua remain separate 

 from the olfactory capsules and connect anteriorly with the 

 suprarostral or labial cartilages. During metamorphosis the 

 labial cartilages and the anterior ends of the cornua disappear 

 in front of the olfactory capsules (Fig. 82). 



The formation of the bony . elements of the skull occurs 

 relatively late in the frog. As a matter of fact, the derm or 

 membrane bones appear before those which are formed in the 

 cartilage cranium, but they will be described later. There 

 are, in the frog's skull the following elements formed as carti- 

 lage bones in the original cranium. 



(a) The exoccipitals (lateral occipitals} which form from the 

 posterior parts of the occipital cartilage and auditory capsule; 

 the occipital condyles and the median dorsal and ventral parts 

 of the occipital region remain cartilaginous. 



(6) The prootics, which form from the anterior parts of the 

 auditory capsules and the parts of the basal plate and orbital 

 region adjacent to the auditory capsules. 



(c) The columellce, whose development has been described in 

 another place. 



(d) The ethmoids which form as vertical elements in the 

 anterior part of the inner w r all of the orbit; later the two eth- 

 moids unite above and below, forming a band-like element 

 around the cranium. This is often known as the sphenethmoid 

 or orbito-sphenoid. 



In the palato-quadrate cartilage, bone appears only in the 

 region of the articulation with the lower jaw (see below). 

 This region does not form a distinct element of the skull, how- 

 ever, but unites with a membrane bone, the two together 

 forming the quadrato-jugal. 



With the exception of the ethmoids, these elements are all 

 present by the end of metamorphosis: the ethmoids form some 

 weeks later. 



