ON THE VERTEBRATE SKELETON. 257 
tion and ossification, do not precisely follow the same route. In the centrums 
of the dorsal and cervical vertebra of the chick chondrification is centripetal: 
it begins from two poiuts 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 centruin. 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 Veriebre of the Trunk.—The ossified parts of the 
abdominal vertebre of osseous fishes answer to ¢, centrum; ”, neurapo- 
physes ; 2 s, neural spine ; p, parapophyses; pl, 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 vertebra unanchylosed 
to their centrums in a large 
sea-perch (Centropristis gigas, 
O.) in which the five terminal 
heemal 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- Ossified parts of abdominal vertebra, hers 
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 hemal 
spines are in like manner moveably articulated to the hemal arches. These 
* Meckel, Archiv fiir die Physiologie, Bd. i. (1815) t. vi. fig. 1. 
