THE LATER DEVELOPMENT OF THE FROG 213 



composed of the occipital and mesotic cartilages and the 

 parachordal plate, is known as the basal plate. 



By the time the tadpole has reached a length of about 14 

 mm. the chondrocranium has acquired the form shown in 

 Fig. 80, B. A still later stage is illustrated in Fig. 81. Com- 

 parison of these two figures brings out most of the facts of 

 later development. We need therefore mention specifically 

 only a few details. No traces of the notochord finally remain; 

 it is partly replaced by, and partly converted into cartilage of 

 the basal plate. The occipital region slowly extends vertically 

 forming the hinder wall of the cranial cavity, and fuses exten- 

 sively with the auditory capsule. Finally the occipital cartilage 

 extends dorsally around the nerve cord, enclosing the foramen 

 magnum. In the frog the occipital cartilage shows no definite 

 indications of its vertebral origin. 



The auditory capsule becomes more complete externally, 

 remaining open into the cranial cavity by a large foramen. 

 From the inner surface of the capsule cartilage grows in, sur- 

 rounding the elements of the membranous labyrinth previously 

 described. On the outer side of the capsule an opening is 

 formed, the secondary fenestra vestibuli, which becomes plugged 

 by the movable operculum. In connection with the ear we 

 described above the development of the plectrum or columella, 

 and its connection later with the annular cartilage in the 

 superficial tympanic membrane. We should repeat that the 

 columella is not related with the elements of the visceral 

 skeleton in the frog. 



In the orbital region the trabecula? gradually grow across 

 the basicranial fontanelle, closing it and forming the floor of 

 this part of the cranial cavity. They also extend vertically 

 forming the lateral walls of the cranial cavity, separating it 

 from the orbits; these walls are perforated only for the passage 

 of nerves and blood vessels. Anteriorly, cartilages from the 

 trabeculse also extend dorsally across the mid-line forming a 

 narrow dorsal bridge. The large supracranial fontanelle 

 between this bridge and the supraoccipital region is not 

 closed by cartilage. 



