THE SKELETAL SYSTEM 615 



chordal plate to unite with the auditory capsule ventrally, both anteriorly 

 and posteriorly. 



The occipital cartilage is a continuation of the mesotic cartilage 

 which fuses with the auditory capsule, and leaves a small opening 

 through which the IX and X cranial nerve pass. This opening is called 

 the jugular foramen. 



The basal plate is the name given to the floor of the posterior por- 

 tion of the cranium, which consists of occipital and mesotic cartilages 

 together with the parachordal plate. 



The occipital cartilage extends dorsally around the neural cord to 

 form the foramen magnum. 



The auditory capsule remains open into the cranial cavity internally 

 by a large foramen, but closes externally. 



The trabeculae now grow across the basicranial fontanelle so that 

 it becomes entirely closed. This closed portion is the floor of the cranial 

 cavity. The trabeculae then extend laterally and form the lateral walls 

 of the cranial cavity, thus separating the cavity from the orbits. 



Cartilages from the trabeculae also extend dorsally across the mid- 

 line in the anterior region, thus forming a narrow dorsal bridge, leaving 

 a large supracranial fontanelle between this bridge and the supra- 

 occipital region. 



The internasal septum extends dorsally and becomes the anterior 

 wall of the cranial cavity, while the trabecular cornua remain separate 

 from the olfactory capsules, but connect anteriorly with the suprarostral 

 or labial cartilages. During metamorphosis, however, both labial car- 

 tilages and anterior ends of the cornua disappear in front of the olfactory 

 capsules. 



True bones form late in the frog, the following being the more im- 

 portant ones which have developed from cartilage : 



Exoccipitals, or Lateral Occipitals. These form from the posterior 

 portions of the occipital cartilage and auditory capsule. The occipital 

 condyles themselves as well as the median dorsal and ventral portions 

 of the occipital region remain as cartilage. 



Pro-otics. These form from the more anterior portion of the audi- 

 tory capsules as well as from the basal plate and orbital region. 



Ethmoids. These form in the anterior portion of the wall of the 

 orbit. They then unite both above and below so as to form a band around 

 the cranium, often also called the sphenethmoid or orbito-sphenoid. 



Quadrato-jugal. The palato-quadrate cartilage forms bone only in 

 the region of the lower jaw. Then a connection is formed with a mem- 

 brane bone and these two together form the quadrate- jugal. All these 

 bones form before metamorphosis, the ethmoids alone developing some 

 weeks after metamorphosis has taken place. 



The Visceral Skeleton. In the mandibular and hvoid arches as well 



