268 OUTLINES OF CHORDATE DEVELOPMENT 



remainder of the embryonic region thus far formed, the endo- 

 derm and mesoderm are united in a secondary median fusion 

 continuous with the cells of the primitive knot. Very soon 

 the mesoderm and endoderm separate again and the median 



a.o. 



F.G. 



-pr.gr. 



FIG. 104. Chick embryo with three pairs of somites (about twenty-three 

 hours). Dorsal view. From Lillie (Development of the Chick), a.c.v., Amnio- 

 cardiac vesicle; a.o., inner margin of area opaca; F. G., fore-gut; N'ch., notochord; 

 n.F., neural fold; pr.gr., primitive groove; s.l, s.2, s.3, first, second and third 

 somites. 



cells, then associated with the mesoderm, become arranged as 

 a small longitudinal rod, the notochord (Fig. 102). Since the 

 germ layers are fused in the primitive knot, and redifferen- 

 tiate anteriorly out of it, it is of little consequence whether 

 the notochord is regarded as of mesodermal or endodermal 

 origin; it is more closely associated with cells which later are 



