DEVELOPMENT. 679 



the progress of its development, it is found to become inclosed in a 

 membranous sheath, which at length acquires a fibrous structure, com- 

 posed of transverse annular fibres. The chorda dorsahs is to be regarded 

 as the azygos axis of the spinal column, and, in particular, of the future 

 bodies of the vertebrae, although it never itself passes into the state of 

 hyaline cartilage or bone, but remains inclosed as in a case within the 

 persistent parts of the vertebral column which are developed around it. 

 It is permanent, however, 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 amphioxus, in which the notochord persists through life 

 and there are no vertebrae, through the lampreys in which there are a 

 few scattered cartilaginous vertebrae, and the sharks, in which many of 



ss 



N * 



FIG. 466. Embryo chick (4th day), viewed as a transparent object, lying on its left side (mag- 

 nified). C H, cerebral hemispheres; FB, fore-brain or vesicle of third ventricle, with Pn, pineal 

 gland projecting from its summit; MB, 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; Pa, pneumogastric nerve, pass- 

 ing towards tne fourth visceral arch ; i v, investing mass; ch, 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 down- 

 wards, and then terminates in a point; Ht, heart seen through the walls of the chest; MP, muscle 

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

 and hand; HL, hind-limb, as yet a shapeless bud, showing no differentiation. Beneath it is seen 

 the curved tail. (Foster and Balfour.) 



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: 



Thje rnesoblastic somites, which have been already mentioned (p. 

 666); send processes downwards and inwards to surround the notochord, 



