EMBRYOS OF ABOUT TWENTY HOURS INCUBATION 



37 



chordal plate or head process, which is temporarily united with the entoderm 

 (Fig. 30). 



Head 

 process 



A 

 Primitive 



Primitive 

 groove 



C 

 Area opaca 



iU Mesoderm 



Area 

 pelhicida 



Primitive 

 C streak 



Blood island 



Fig. 30. — Dorsal surface view of a twenty-hour chick embryo showing primitive streak and 

 extent of mesoderm (after Duval). X 17. The lines A, B, and C indicate the levels of the corres- 

 ponding sections shown in Fig. 31. 



Ectoderm 



Neural plate 



Mesoderm 



Ectoderm 



^^5|^^iS^iS*i»^f*^^ 



Notochordal plate Entoderm 



Primitive knot l,jWr<4cN « Kcot^ 

 B 



Mesoderm 

 Ectoderm 



Enliiderm 

 Primitive groove 



Mesoderm Entoderm 



Fig. 31. — Transverse sections through the embryonic area of a twenty-hour chick. X 165. ^, through 

 the head process; B, through the primitive knot; C, through the primitive streak. 



A transverse section through the primitive streak at twenty hours (see guide 

 line C, Fig. 30) shows the three germ layers distinct laterally (Fig. 31 C). In the 



