648 EMBRYOLOGY. 



that the axial skeleton undergoes a segmentation into separate like 

 portions, which are situated one behind another ; to accomplish this, 

 remnants of the parental tissue do not chondrify, but become, 

 between the bodies of the vertebrae, the intervertebral discs, and, 

 between the arches, the ligamenta intercruralia, etc. 



10. The segmentation of the vertebral column has been dependent 

 in its origin upon the segmentation of the musculature, and has 

 been effected in such a way that skeletal segments and musculai 

 segments alternate with one another, and that the longitudinal 

 muscle-fibres, which lie alongside the axial skeleton, are attached 

 by their anterior and posterior ends to two [adjacent] vertebrae and 

 are capable of moving them upon each other. 



11. The chorda is more or less restrained in its growth by the 

 cartilaginous bodies of the vertebrae surrounding it, and degenerates 

 in different ways in the different classes of Vertebrates ; in Mammals 

 the part located in the body of the vertebra is completely obliterated, 

 whereas a remnant of it is preserved between vertebrae and becomes 

 the jelly-core of the intervertebral disc. 



12. The cartilaginous vertebral column is converted in most 

 Vertebrates into a bony one, by the breaking down of the carti- 

 laginous tissue, which begins at different places, and its replacement 

 by bony tissue. (Formation of bone-nuclei or centres of ossification.) 



13. The ossification of each cartilaginous vertebral fundament in 

 Mammals and Man proceeds from three centres, from one in the 

 body and one in each half of the arch, to which subsequently 

 certain accessory centres are added. 



14. With each vertebral segment there is associated a pair of ribs, 

 which arise by a process of chondrificatioii in the layers of tissue 

 which separate the muscle -segments (the ligamenta intermuscularis). 



15. In Man the various regions of the vertebral column are 

 produced by metamorphosis of the vertebral and costal fundaments. 



(1) The thoracic part of the vertebral column (dorsal vertebrae) 



is characterised by the following peculiarities : the ribs 

 attain to complete development ; a part of them become 

 expanded at their ventral ends, and united to form the 

 two sternal bars, by the fusion of which the unpaired 

 sternum is produced. (Fissura sterni, an arrested forma- 

 tion.) 



(2) In the cervical and lumbar regions of the column the funda- 



ments of the ribs remain small, and fuse with outgrowths 

 from the vertebrae the transverse processes to form 



