PISCES 



161 



plates are formed laterally, the sphenoid plates. Around the 

 auditory organ, beginning basally, a cartilaginous capsule is 

 formed, and similarly the nose on each side is invested in a dome- 

 shaped capsule. The parachordals expand up the side of the 

 brain and fuse with the auditory capsules. The trabeculae, 

 gradually rotated into the plane of the parachordals, likewise 

 expand and fuse with the nasal capsules and the sphenoid plates. 

 The result is a cartilaginous trough for the brain carried 

 forwards as a rostrum, and to which the nasal capsules and the 

 auditory capsules are fused as wings. The roof is invested only 



And. caps. 

 Rostrum Nasal caps. Fontanelles \ For. mag. Vert. col. 



Lab. cart. 



Up. Low. Hyo- 

 jaw jaw mand. 

 Spir. cart. 



Hyoid Branch, arch Pectoral girdle 



FIG. 80. Raia. Diagrammatic view of the chondrocranium, visceral 

 arches, and the pectoral girdle of the adult skate. 



between the auditory capsules and by a narrow bar between 

 the nasal capsules, and thus it presents anterior and posterior 

 fontanelles. The brain case opens into the neural canal by 

 the foramen magnum. As it develops, the cartilage of the 

 brain case surrounds the nerves emanating from the brain, 

 and nerve foramina are thus formed. 



The series of arches, called visceral arches, developed 

 around the anterior end of the alimentary canal are the mandi- 

 bular, hyoid, and five branchial arches, all paired. The mandi- 

 bular arch is divided to form the upper jaw or palatoquadrate 

 cartilage and the lower jaw or Meckel's cartilage. The hyoid 

 arch yields the hyomandibular cartilage which suspends the jaw 

 cartilages to the auditory capsule. The lower part of the hyoid 

 is resolved into three segments, epi-, cerato-, and hypo-hyal. 



