BLASTOCOEL 



NTERNAL MIGRATION OF CELLS 



POSTERIOR 



Fig. 120. A simplified version of early gastrulation in the chick. Near the 

 posterior border of the blastoderm, cells migrate into the blastocoel and a layer 

 of cells spreads in an anterior and lateral direction. The result is a sheet of 

 endoderm underneath the blastoderm. 



PRESUMPTIVE HEAD PROCESS 

 NEURAL PLATE (NOTOCHORO) 



hensen's NOOE 



PRIMITIVE STREAK 



iRCHENTERON 



ECTODERM 



MESODERM 



ENDODERM 



' 1 1 1 1 1 1 1 1 1 1 1 1 1 1 /V«' 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 



Fig. 121. The primitive streak in longitudinal section. It is continuous with 

 the endoderm below. The anterior end of the streak is Hensen's node. From 

 this an outgrowth, the head process, develops into the notochord. Above the 

 notochord is the presumptive neural plate. Posteriorly the primitive streak joins 

 the ectoderm, mesoderm, and endoderm. Compare with Figure 122, which 

 shows a cross section of a primitive streak. 



MIGRATION 

 or CELLS 



PRIMITIVE GROOVE 



PRIMITIVE STREAK 



PRESUMPTIVE NOTOCHORD 



Fig. 122. The primitive streak in cross section. The migration of cells is 

 shown by arrows. Cells from the surface layer migrate toward and into the 

 primitive, streak. From it cells migrate further into the mesoderm and possibly 

 into the endoderm. The dotted circle represents cells which give rise to notochord. 



207 



