380 ZOOLOGY. 



On the other hand, while the brain of insects is a single 

 pair of ganglia like those of the rest of the body, the differ- 

 ent ganglia forming the brain of Vertebrates are concen- 

 trated in the head alone ; still the different pairs of nerves 

 sent off from the base of the brain are homologous with 

 the spinal nerves, sent off at intervals corresponding to each 

 vertebra. 



There are two theories of the composition of the skull. 

 That of Oken, Goethe, and of Owen, who believed that the 

 skulls of the bony fishes and mammals were composed of 

 three or four segments. . It should be noticed that these 

 views are based on an examination of highly specialized ver- 

 tebrates. From a study, however, of the more generalized 

 types of fishes (such as the sharks), and the embryos of ver- 

 tebrates belonging to different groups, the old vertebrate 

 theory of the skull has been discarded, and the view of Ge- 

 genbaur, confirmed by Salensky, is probably nearly the cor- 

 rect one. As stated by Gegenbaur : 



1. The skull is comparable to a portion of the vertebral 

 column, which contains at least as many vertebral segments 

 as there are branchial arches. This view is borne out by the 

 following facts : 



a. The notochord, which forms the foundation of the 



vertebral column, passes through the cranium in the 

 same way as it passes through the vertebral column. 



b. All the nerves which pass out of the base of the 



skull (or that portion traversed by the notochord) 

 are homologous with the spinal nerves. 



c. The difference between the skull and vertebral col- 



umn consist of secondary adaptations to certain con- 

 ditions, which are external to the skull, and are 

 partly due to the development of a brain. 



2. The skull may be divided into two regions, a vertebral 

 portion and an anterior evertelral portion, lying beyond 

 the end of the notochord. 



3. The number of vertebraB which enter into the forma- 

 tion of the skull are nine at least (according to Salensky, in 

 the sturgeon, seven); the exact number is immaterial.* 



* The number of mesoblastic somites concerned in the formation of 

 the skull is nine. 



