THE SKELETOX 



417 



a separate component. Schauinsland has interpreted it as the 

 homologue of the haemal arches of reptilia {e.g., Sphenodon). 



The membrane represents not only the future bony parts 

 but the ligaments and periosteum as well. Hence we find that 

 the successive membranous vertebrae are not separate structures 

 but are united by membrane, i.e., condensed mesenchyme, and 

 are distinguishable from the future ligaments at first only by 

 greater condensation. In the stage of Fig. 233, chondrification 

 has already begun in the vertebral body, hence there is a sharp 



/v'a- 



Fig. 235. — Median sagittal section of the cervical region at 



the end of the sixth day of incubation. (After Froriep.) x 40. 



b. C, Basis cranii. iV. L. 1, 2, 3, First, second, and third 

 intervertebral ligaments, s. n. b. 1, 2, 3, 4, First, second, third, 

 and fourth subnotochordal bars (hypocentra). v. C. 3, 4, 

 Pleurocentra of third and fourth vertebrae. 



distinction in this region l^etween the vertebral bod}^ and inter- 

 vertebral discs. The centers of chondrification, however, grade 

 into the membranous costal processes and neural arches. 



The vertebral segmentation has now become predominant as 

 contrasted with the primitive somitic. 



The development of the vertebrae during the fifth day com- 

 prises: (1) Fusion of successive caudal and cephalic divisions of 



