Traces of Unity, &c. 59 



nothing more than the very softest of soft notochords.* 

 In many cartilaginous fishes now living, as in the 

 sturgeon, a wide continuous canal runs through the 

 bodies of the spinal vertebrae from one end of the spinal 

 column to the other : and, besides this, the bodies them- 

 selves are cut up into segments by deep fissures running 

 in an antero-posterior direction. And even in mammals 

 some of these changes in the bodies of the spinal 

 vertebras are repeated, for the bodiless upper cervical 

 vertebra, or atlas, approaches evidently to the bodiless 

 dorsal vertebra of the chelonian, and the division of the 

 lumbar vertebra into two lateral halves by a deep 

 groove which occurs in the sea-cow (Manatus), and, in a 

 lesser degree, in man himself, and in many other 

 mammals, may be an exemplification of the same seg- 

 mentation as that to which reference has just been made 

 as existing in the cartilaginous fish. 



Very often, ajso, the bodies of the cranial vertebrae 

 are represented by a mere plate of bone ; and now and 

 then even this plate may be wanting. Thus, in some of 

 the seals there is a large aperture in the dry skull where 

 some of these bodies should have been >an aperture 

 which during life is only closed by soft tissue ; and thus 

 again, in the cyclostome fishes, as the myxine and 

 lamprey, the floor of the skull is in the main formed by 

 two cartilaginous ridges homologous with the trabeculse 

 of the embryonic skull which separate fora short distance 

 and then re-unite so as to enclose a space which is occu 

 pied by a layer of cartilage not much thicker than paper. 



The cyclostome fish, which occupies an intermediate 



position between fishes and reptiles, represents a very 



rudimentary phase of development ; the seal may be 



said to be the least developed of mammal forms ; and 



* Owen: Comp. Aqat. and Phys. of the Vertebrata ; vol. i., p. 197. 



