THE SKULL 141 



meet each other both above and below the notochord, which 

 they now completely surround. The two parachordals soon fuse 

 together to form the basilar plate, which, with the trabeculae, 

 forms a firm cartilaginous floor to the brain case. At their 

 hinder ends the parachordals grow upwards to form the side 

 walls of the cranium, and a little later bend inwards so as to 

 meet each other above the brain, and complete the occipital 

 part of the cranium. Further forwards the pituitary foramen 

 becomes closed by a thin plate of cartilage, and the lateral 

 margins of the parachordals and trabeculre grow upwards so as 

 to form the side walls of the skull, the roof remaining im- 

 perfect in this region. 



The first bone to be developed is the parasphenoid. The 

 exoccipitals, the frontals and parietals, which are at first 

 separate, and other bones soon follow ; and by the time the 

 metamorphosis is complete and the tail absorbed, all the bones 

 of the adult cranium are present, except the sphenethmoid, 

 which does not appear till some months later. 



b. The Sense Capsules. The cartilaginous auditory capsules 

 appear in tadpoles of about 12 mm. length as thin shells of 

 cartilage investing the auditory vesicles. They are at first 

 quite independent of the cranium, but before the completion of 

 the opercular folds they fuse with the upgrowing parachordals 

 to form part of the side walls of the skull. The pro-otic appears 

 about the time of completion of the metamorphosis. 



The optic capsules are thin shells of cartilage, forming part 

 of the sclerotic coats of the eyes. They arise about the same 

 time as the auditory capsules ; and, unlike the other sense 

 capsules, they remain distinct from the cranium throughout life, 

 in order to secure mobility of the eyeballs. 



The olfactory capsules are from their first appearance very 

 closely connected with the anterior ends of the trabeculse, which 

 grow up between them to form the median vertical internasal 

 septum. They develop later than the auditory and optic capsules. 



c. The Visceral Skeleton. This consists of a series of carti- 

 laginous hoops developed within the visceral arches, and forming 

 a framework which surrounds and stiffens the walls of the 

 pharynx. Each hoop consists of right and left halves, which 

 ai independent at their dorsal ends, but fused or closely con- 

 nected ventrally. There are in all six of these hoops or bars 

 forming the oral (mandibular) arch, hyoidean arch, and the 



