THE STUDY OF CHICK EMBRYOS 



C 

 Area opaca 



FIG. 3 o.-Dorsal surface view of a twenty-hour chick 

 head process and extent of mesoderm (after Duval). X 17. 

 the levels of the corresponding sections shown in Fig. 31. 



lines A. B, and 



Ectoderm 



Mesoderm 



Ectoderm 



Mesoderm 



Ectoderm 



Neural plate 



'Notochordal plate Enloderm 



Primitive knot 



B 



Primitive groove 



"/ ' S</ "" J \ 



Mesoderm Entoderm 



FIG. 31. Transverse sections through the embryonic area of a twenty-hour chick. 



X 165. A, through the head process; B, through the primitive knot; C, through the 

 primitive streak. 



