418 CHORDATA. 
the second and third form the ¢yroid, and probably the 
arytenoid cartilages of the larynx. 
THe VERTEBR#.-—-A typical vertebra consists of a 
centrum or main axis, above which is a bony xeural arch 
covering in the spinal cord. It is often surmounted by 
a more or less prominent median neural spine. From each 
side of the neural arch there usually protrudes a lateral 
process known as the ¢ransverse process. In the anterior 
part of the vertebral column the vertebra usually bears a 
rib, which is articulated to the centrum by its head or 
capitulum and to the transverse process by its tuderculum. 
The rib may, however, become completely fused on to the 
vertebra (cervical), or it may be attached only to the 
transverse process, or may become fused with the transverse 
process (lumbar and sacral). In the region behind the 
sacrum there is often a hemal arch, but in mammals this is 
only found in a few types in the form of chevron bones 
which articulate Jefween the vertebre. In dAZammatia the 
centra have epiphyses or caps of bone, and these are usually 
flat, though they may be opisthoccelous in some of the 
cervicals. At the front and hind-end are anterior and 
posterior zygapophyses which serve as articulations between 
the vertebree. 
The vertebre are usually divided into—(1) cervical, 
(2) thoracic, (3) lumbar, (4) sacral and (5) caudal. The 
cervicals are defined as lying between the skull and the 
first thoracic, or the first vertebra that has a pair of ribs 
which meet the sternum. The thoracic vertebre bear ribs 
which meet the sternum. In all the higher Vertebrata the 
sternum is formed from the fusion of the distal ends of the 
ribs. 
Development.— The embryo has a notochord, around which is 
formed a continuous mesoblastic membranous or skeletogenous sheath. 
This sheath extends dorsally round the neural tube ( cranium). 
Paired masses of cartilage then appear above and below in the sheath. 
Their bases fuse across from side to side and dorso-ventrally to form the 
cartilaginous centrum, and the dorsal arches grow up round the spinal 
cord to form the neural arch. Ossification then takes place, there 
being usually several centres of ossification. 
APPENDICULAR, — All Vertebrata above Cyclostomata 
(and exceptions) have two pairs of limbs and limb-girdles, 
an anterior or pectoral and a posterior or pelvic. 
