DEVELOPMENT OF THE SPINAL COLUMN. 675 



FIG. 265i 



11 



Transverse section through the dorsal region of an embryo chick of forty- 

 five hours. (Foster and Balfour.) A. Epiblast. MX. Medullary canal. 

 P.v. Protovertebrse. W.d. Wolffian duct. p.p. Pleuro-peritoneal cavity. 

 S.o. Somatopleure. S.p. Splanchnopleure. v.v. Vessels, a.o. Aorta. B. 

 Mesoblast. C. Hypoblast. o.p. Line of union of opaque and pellucid 

 areas, w. Spheres of the white yolk. 



