ON THE VERTEBRATE SKELETON. 



257 



tion and ossification, do not precisely follow the same route. In the centrums 

 of the dorsal and cervical vertebrae of the chick chondrification is centripetal: 

 it begins from two points at the sides and proceeds inwards, the middle line 

 of the under surface of the primitive notochord resisting the change longest. 

 But, when the lateral cartilages have here coalesced, ossification begins at 

 the middle line and diverges laterally ; the primitive nuclei of the bony centres 

 appearing as bilobed ossicles, and its direction is centrifugal. The lobes 

 ascend to embrace the shrivelled remnant of the chorda, like the hollow ver- 

 tebral centres in fishes. Only in the sacral vertebrae has ossification been 

 seen to begin from two distinct points at the middle line. The bases of 

 the separately ossifying neurapophyses extend over much of the centrum, 

 and soon coalesce with it. In reptiles a greater proportion of the centrum 

 is ossified from an independent point, and the bases of the neurapophyses 

 often remain permanently distinct and united to the centrum by suture. In 

 mammals, as in fishes, the centrum is ossified from an anterior and posterior 

 centre, establishing the articular surfaces, as well as from an intermediate 

 point. This is considerably overlapped by the bases of the neurapophyses, 

 before they coalesce with the centrum. The three primitive parts of the 

 centrum remain longest distinct in the cetacea. The body of the human 

 atlas is sometimes ossified from two, rarely from three, distinct centres placed 

 side by side *. From these ascertained diversities in the mode of formation 

 of the central element of the vertebra, it will be seen how little developmental 

 characters can be relied on as affecting the determination of homologous parts. 

 General Characters of Vertebrae of the Trunk. — The ossified parts of the 

 abdominal vertebrae of osseous fishes answer to c, centrum ; n, neurapo- 

 physes ; n s, neural spine ; p, parapophyses ; pi, pleurapophyses ; and a, ap- 

 pendages (fig. 17). 



The neurapophyses com- Fig. 17. 



monly coalesce with their re- 

 spective centrums ; except in 

 the case of the atlas, where the 

 neural arch is sometimes quite 

 separated from the centrum, 

 and wedged between those of 

 the occiput and second verte- 

 bra. I have found also the 

 neurapophyses of the two last 

 caudal vertebrae unanchylosed 

 to their centrums in a large 

 sea-perch (Centropristis gigas, 

 O.) in which the five terminal 

 haemal arches and spines re- 

 mained similarly distinct, and 

 articulated with the centrums 

 below. In the carp and pike, 

 the primitive independence of 

 both neurapophyses and par- 

 apophyses is more general and 

 longer maintained. In the le- 

 pidosiren the vertebral bodies are not developed, the notochord being per- 

 sistent ; but the peripheral vertebral elements are well-ossified : the neur- 

 apophyses in this fish remain distinct from the neural spines ; and the haemal 

 spines are in like manner moveably articulated to the haemal arches. These 



* Meckel, Archiv fur die Physiologie, 13d. i. (1815) t. vi. fig. 1. 



Ossified parts of abdominal vertebra, Fish. 



