G08 ANATOMY OF VERTEBRATES. 



belltim is the last part which is formed by reflection upon the 

 upper and fore part of the epencephalon, A. 



The mode of developement of the cartilaginous cranium is 

 described at pp. 71-74. In the Perch a layer of cartilage-cells 

 beneath the fore-part of the head is continued down on each side 

 into the front border of the inferior transverse mouth : a second 

 cartilaginous arch extends from the side of the cranium, behind 

 the eyes, and supports the hinder and more prominent border of 

 the mouth : a delicate cartilaginous filament from each side of the 

 occiput seeks an attachment with the basis of the rapidly vibrating 

 pectoral fin, and proceeds to curve beneath the cardiac chamber. 

 Between this basis of the scapular arch and the mandibular arch 

 are discernible several smaller arches, beneath the large ear-sacs, 

 of which three are conspicuous as f branchial arches,' but the 

 foremost acquires the most decided gristly structure, and is 

 proximally attached to the origins of the mandibular arch : it 

 becomes the hyoid arch. The first and second inferior or haemal 

 arches, called ' maxillary ' and e mandibular,' rotate forward upon 

 their piers, or points of attachment, and from being vertical 

 become more and more oblique, until the opening of the mouth is 

 brought to the fore-part of the head, and becomes terminal in 

 position : the third, or hyoid, arch, in a minor degree, takes the 

 same forward inclination : the arches between this and the 

 scapular one are monopolised by the branchial organs, which are 

 transitory or undeveloped in the higher Haematocrya. Ossifica- 

 tion in the proto-cranial cartilage begins in the four pairs of 

 neurapophyses, answering to the four hremal arches below, and to 

 the four primary divisions of the encephalon : the four vertebral 

 segments composing the head are as instructively illustrated by 

 the developement of the skull as by that of the brain. 



The scales are formed late in all Osseous Fishes : their integu- 

 ments remain smooth and lubricous, as in the Dermopteri, some 

 time after the disappearance of the vitellus. 



After the formation of the embryonal, continuous fin-fold 

 blastema accumulates in its dorsal, anal, and caudal regions ; 

 and, as the rays are here formed, the intervening membrane 

 begins to be absorbed. The fin-rays (dermo-neurals and -ha3inals) 

 commence near the free border, and elongate by approaching the 

 neural spines : they there meet the inter-neurals and -hremals, 

 which grow in the opposite direction. During the formation of 

 the caudal rays, the end of the notochord, in the Pike, Perch 

 and Salmon, bends upward, or c neurad : ' the heterocercal type 

 succeeds the protocercal one, and is followed by the resumption 

 of symmetry under the more advanced e homocercal ' condition. 



