STRUCTURE OF FISHES. 291 



of the body of each vertebra.* Frequently the sides do not 

 quite meet, and the leaves, which are developed from the 

 upper surfaces of the vertebrae, now form arches over the 

 spinal cord, and are united above by spinous processes. Yet 

 the whole skeleton in these fishes remains in the incipient 

 stage of ossification, being more or less cartilaginous; and 

 where the ossific process has begun, it has not advanced the 

 length of producing union between the pieces formed from 

 the separate centres of ossification. Where they meet with- 

 out uniting, they form no sutures', but overlap one another. 

 Thus the bony structures are detached, and often complete- 

 ly isolated; affording to the physiologist an opportunity of 

 studying the earlier stages of this interesting process, and 

 marking with distinctness the number of the elements of 

 each bone, and the relative situations of their centres. This 

 knowledge is more especially of importance towards under- 

 standing the formation and connexions of the bones of the 

 head, which are very numerous and complicated; and the 

 investigation of which has been prosecuted with extraordi- 

 nary diligence by Geoflfroy St. Hilaire and other continental 

 zootomists. 



It is here, more especially, that we obtain the clearest evi- 

 dence of the derivation of the cranial bones from vertebrae 

 analogous to those of the spine. The occipital bone, in par- 

 ticular, corresponds to a spinal vertebra in all its essential 

 elements. In many fishes, the body of this bone, being 

 lengthened out to form the posterior part of the basis of the 

 skull, becomes the basilar portion. We find, on its posterior 

 surface, the same cup-like cavity as in the true vertebrae, 

 and it is joined to the next vertebra in the same manner as 

 the spinal vertebrae are joined to each other. Its crest has 



• A small aperture still remains, establisliing- a communication between 

 the cavities the whole length of the spine. This is supposed to be dcsig-ned 

 to obviate the compression of the fluid in the different cells or cavities during- 

 the motions of the spine. The vertical sections, Fig. 189 and 190, of two 

 contiguous vertebra in different fishes, will convey an idea of this gradation 

 of development. 



