518 CHORD ATA. 



a breast bone or sternum (Amniotes, fig. 558). The sternum is 

 a derivative of the ribs. In development the ventral ends of the 

 ribs of a side fuse and then these fused tracts of the two sides 

 unite to form the sternum. 



The haemal arches lie internal to the longitudinal muscles of the body, 

 and in the trunk region they lie in the same position just beneath the 

 peritoneum. These are hcemal ribs and are found only in teleosts and 

 ganoids. The ribs of all other vertebrates (elasmobranchs, amphibia, 

 amniotes) are morphologically different and are called lateral or pleural 

 ribs. They develop independently of the vertebral column in a horizontal 

 connective-tissue septum which extends out through the longitudinal mus- 

 cles from the axial skeleton to the skin, dividing the musculature into 

 dorsal (epaxial) and ventral (hypaxial) portions (fig. 89). In the elasmo- 

 branchs these pleural ribs are attached to the haemapophyses, in the 

 others to the transverse processes (diapophyses), which arise from the 

 neurapophyses, and parapophyses, which arise from the vertebral centres. 

 In the caudal region, often also in the cervical, lumbar, and sacral regions, 

 the pleural ribs and dia- and parapophyses fuse to form lateral processes. 

 These occur concurrently with haemal arches in the tails of many Am- 

 phibia and reptiles and some mammals, forming the chevron bones which, 

 as in fishes, enclose the caudal blood-vessels. The presence of intercalaria 

 in cyclostomes, sharks, and ganoids indicates that primitively a double 

 vertebra arose in each somite. Paleontological and embryological re- 

 searches on reptiles support this view. 



In most vertebrates either the basal ends of the arches broaden 

 out around the notochord and fuse with one another, or perichordal 

 cartilages arise independently, furnishing in either case firm sup- 

 ports, the vertebral bodies, or centra, for the system of arches. 

 These increase in size at the expense of the notochord on the in- 

 side, sometimes leading to its almost complete obliteration, as in 

 the mammals; in others, as the fishes, the reduction is less com- 

 plete. The fishes have awphicoele vertebra} (fig. 557), that is, the 

 centra are hollow at either end. In these cups the notochord 

 exists even in the adult, and when small connecting portions ex- 

 tend through the centra the notochord takes the form of a rosary 

 with alternating enlargements and contractions. 



Histologically the vertebral column may be either cartilage or 

 bone; usually it is first formed in cartilage, which is later replaced 

 by bone. If the ossification does not occur, the column remains 

 cartilaginous; if incomplete, cartilage and bone appear side by side. 

 iSince these histological differences are combined with varying de- 

 grees of persistence of the notochord and with modifications in the 

 form of the vertebrae and their processes, there results an extraor- 

 dinary variety in the appearance of the vertebral column. 



