374 STRUCTURE OF VERTEBRATES. 



or less intimately associated mandibular and hyoid arches, 

 (d) from membrane or investing bones of extrinsic origin 

 (see the table on page 375). 



The brain-box is cartilaginous in Cyclostomata and Elas- 

 mobranchs; centres of ossifications and investing bones 

 begin with the Ganoid fishes and are numerous in Teleos- 

 teans. Above Elasmobranchs, in short, the gristly brain-box 

 is more or less thoroughly replaced or covered by bones. 

 In the individual development there is a parallel progress. 



Theory of the Skull. 



Near the beginning of this century, Oken and Goethe independently 

 propounded what is known as the vertebral theory of the skull. They 

 imagined that the skull was comparable to three or four vertebral bodies, 

 and that the bones of each of the regions were comparable to the parts 

 of a vertebra. Thus the basi-occipital, the two ex-occipitals, and the 

 supra-occipital of the hindmost region of the skull, were held to corres- 

 pond to the centrum, the neural arches, and the neural spine of a 

 vertebral body. 



But this undoubtedly suggestive theory was disproved by the subse- 

 quent discoveries of comparative anatomy and embryology. Huxley 

 gave it a death-blow, showing that the suggested homologies were false, 

 while Gegenbaur replaced it by what may be called the segmental 

 theory, according to which the skull is the result of about nine primitive 

 segments. Their number is inferred from the distribution of the cranial 

 nerves. 



Before there is any trace of skull, the head consists of nine to eleven 

 primitive segments, like those which in the trunk give rise to myomeres, 

 etc. But the cartilaginous skull which gradually grows is not itself 

 segmented, still less can the primary or secondary bones of the bony 

 skull be referred to definite segments. It is the head which is segmented, 

 not the skull, and the segmentation of the head can be securely appre- 

 ciated only from the early stages when the primitive segments are still 

 demonstrable, though the results thus reached may be corroborated by a 

 study of the nerves, the lateral sense-organs, the ganglia, and the arches. 

 But the matter must still be left for a while in the hands of experts, until 

 greater harmony of conclusion is attained. 



Backbone. — The foundation of the axial skeleton is the 

 notochord, which is formed from endoderm or hypoblast 

 along the dorsal wall of the embryonic gut. In most Verte- 

 brates the backbone develops as the substitute of this noto- 

 chord, but not from it. 



For (i) around the hypoblastic notochord there grows a 

 mesoblastic sheath, which also extends as a membranous 



