n.p. 

 p.s. 



Al 



FIG. 218. Stages in development of Chick. (After MARSHALL.) 



1. Segmentation, superficial view of blastoderm. 



2. Vertical section of blastoderm ; Ep., epiblast ; I.e., lower layer of cells ; s.g.c., seg- 

 mentation cavity ; y., yolk. 



3. Diagrammatic surface view; ./., area pellucida; a.o., area opaca ; n.p., neural 

 groove ; p.s., primitive streak. 



4. Diagrammatic surface view at later stage ; ap., area pellucida ; a.o., area opaca ; 

 m.s., mesoblast segments ; p.s., primitive streak. 



5. Cross section ; s.c., spinal cord ; s.g.) rudiment of spinal ganglia ', JV., notochord ; 

 m.p., mesoblastic plates ; A., aorta ; am., amnion fold. 



6. Embryo ; cb., cerebellum ; F., ear ; ff., heart ; f.L, fore limb ; k.I., hind limb ; y.s., 

 stalk of cut-off yolk sac ; AL, allantois ; E., eye ; c. , cerebrum. 



