276 OUTLINES OF CHORDATE DEVELOPMENT 



~~pr. str. 



FIG. 107. Chick embryo with twelve pairs of somites (about thirty-three 

 hours). From Lillie (Development of the Chick). A. Dorsal view of entire 

 embryo. B. Ventral view of anterior end. A.C.S., Anterior cerebral suture; 

 a.i.p., anterior intestinal portal; Ao., dorsal aorta; F.G., fore-gut; H.B., hind- 

 brain; Ht., heart; M.B., mid-brain; op. Fes., optic vesicle; or.pl, oral plate; 

 pr.str., primitive streak; s.2, s.12, second and twelfth somites; v.ao., ventral aorta; 

 V.o.m., omphalo-mesenteric vein. 



