THE SKELETON 417 
a separate component. Schauinsland has interpreted it as the 
homologue of the hemal arches of reptilia (e.g., Sphenodon). 
The membrane represents not only the ftiture bony parts 
but the ligaments and periosteum as well. Hence we find that 
the successive membranous vertebre are not separate structures 
but are united by membrane, 7.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 
0 
the end of the sixth day of incubation. (After Froriep.) x 40. 
b. C., Basis cranii. i’v. 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 vertebre. 
distinction in this region between the vertebral body and inter- 
vertebral dises. The centers of chondrification, however, grade 
into the membranous costal processes and neural arches. 
The vertebral seementation has now become predominant as 
contrasted with the primitive somitic. 
The development of the vertebree during the fifth day com- 
prises: (1) Fusion of successive caudal and cephalic divisions of 
