NOTOCHORD AND VERTEBRAL COLUMN. 



555 



halves of two intervertebral cartilages, viz. an anterior convex 

 half and a posterior concave half. The vertebrae thus come to 

 be opisthoccelous. A longitudinal section through the vertebral 

 column at this stage is diagrammatically shewn in fig. 320 C. 



To the centre of each of these vertebrae the neural arches, 

 the origin of which was described above, become in the 

 meantime firmly attached ; and grow obliquely upwards and 

 A B CUE 



FIG. 320. DIAGRAM REPRESENTING THE MODE OF DEVELOPMENT OK THE 



VERTEBRA IN THE DIFFERENT TYPES. (From Gegenbaur.) 



A. Ideal type in which distinct vertebrae are not established. 



B. Type of Pisces with vertebral constrictions of the notochord. 



C. Amphibian type, with intervertebral constrictions of the notochord by the 

 intervertebral parts of the cellular sheath. 



D. Intervertebral constriction of the notochord as effected in Reptilia and Aves. 



E. Vertebral constriction of the notochord as effected in Mammalia, the interver- 

 tebral parts of the cartilaginous sheath being converted into intervertebral ligaments. 



c . notochord ; cs. cuticular sheath of notochord ; s. cartilaginous sheath ; v. verte- 

 bral regions ; iv. intervertebral regions ; g, intervertebral joints. 



backwards, so as to meet and unite above the spinal cord. The 

 transverse processes of the vertebrae would seem (Pick) to be 

 developed independently of the arches, though they very soon 

 fuse with them. According to Gotte the transverse processes 

 are double in the trunk, there being two pairs, one vertically 

 above the other for each vertebra. The pair on each side 

 eventually fuse together. 



In the tail haemal arches are formed, which are similar in 

 their mode of development to the neural arches. 



The unconstricted portion of the notochord, which persists in 

 each vertebra, becomes in part converted into cartilage. 



