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. 
. 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. 
e. The difference between the skull and vertebral col- 
umn consist of secondary adaptations to certain con- 
ditions, which are external tothe 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 evertebral portion, lying beyond 
the end of the notochord. 
3. The number of vertebres 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. 
