GENERATION AND DEVELOPMENT. 



271 



at length acquires a fibrous structure, composed of transverse an- 

 nular fibres. The chorda dorsalis is to be regarded as the azygos axis 

 of the spinal column, and, in particular, of the future bodies of the ver- 

 tebrae, although it never itself passes into the state of hyaline cartilage or 

 bone, but remains enclosed as in a case within the persistent parts of the 

 vertebral column which are developed around it. It is permanent, how- 

 ever, only in a few animals: in the majority only traces of it persist in 

 the adult animal. 



In many Fish no true vertebrae are developed, and there is every gra- 

 dation from the ampliioxus, in which the notochord persists through life 



ss 



FIG. 429. Embryo chick (4th day), viewed as a transparent object, lying on its left side (magni- 

 fied). C H, cerebral hemispheres ; F -B, fore-brain or vesicle of third ventricle, with P n, pineal gland 

 projecting from its summit; M B, mid-brain; C b, cerebellum; IV V, fourth ventricle; L, lens; chs, 

 choroidal slit; Cen V, auditory vesicle; s m, superior maxillary process; IF, 2F, etc., first, second, 

 third, and fourth visceral folds; V, fifth nerve, sending one branch (ophthalmic) to the eye, and 

 another to the first visceral arch; VII, seventh nerve, passing to the second visceral arch; G Ph, 

 glosso-pharyngeal nerve, passing to the third visceral arch; P g, pneumogastric nerve, passing to- 

 ward the fourth visceral arch; i v, investing mass; e 7i, notochord; its front end cannot be seen in. 

 the living embryo, and it does not end as shown in the figure, but takes a sudden bend downward, 

 and then terminates in a point; H t, heart seen through the walls of the chest; M P, muscle-plates; 

 W, wing, showing commencing differentiation of segments, corresponding to arm, forearm, and 

 hand; H L, hind-limb, as yet a shapeless bud, showing no differentiation. Beneath it is seen the 

 curved tail. (Foster and Balfour.) 



and there are no vertebral segments, through the lampreys in which there 

 are a few scattered cartilaginous segments, and the sharks, in which many 

 of the vertebrae are partly ossified, to the bony fishes, such as the cod and 

 herring, in which the vertebral column consists of a number of distinct 

 ossified vertebrae, with remnants of the notochord between them. In 

 Amphibia, Eeptiles, Birds, and Mammals, there are distinct vertebrae, 

 which are formed as follows: 



Protovertebrae. The protovertebrcz, which have been already men- 

 tioned (p. 258, Vol. II. }, send processes downward and inward to surround 

 the notochord, and also upward between the medullary canal and the 

 epiblast covering it. In the former situation, the cartilaginous bodies of 



