CHAPTER V 



THE FORMATION OF THE PRIMITIVE STREAK AND 

 THE ESTABLISHMENT OF THE MESODERM 



THE LOCATION AND APPEARANCE OF THE PRIMITIVE STREAK; 

 THE ORIGIN OF THE PRIMITIVE STREAK BY- CONCRESCENCE 

 OF THE BLASTOPORE; THE FORMATION OF THE MESODERM. 



The Location and Appearance of the Primitive Streak. 



The stages of development described in the preceding chapters 

 take place before the egg is laid. The first conspicuous struc- 

 tural feature to make its appearance in the embryo after the 

 laying of the egg is the primitive streak. In eggs that have been 

 incubated about i6*hours the primitive streak is well developed 



cephalic end 



primitive pit 



primitive groove 



primitive ridge 



FIG. 8. Dorsal view ( X 14) of entire chick embryo in the primitive streak stage 

 (about 1 6 hours of incubation). 



as a linear groove flanked on either side by ridge-like thickenings, 

 extending from the inner margin of the area opaca to approxi- 

 mately the center of the blastoderm (Fig. 8). The primitive 

 streak lies in the longitudinal axis of the future embryo. The 

 end adjacent to the area opaca is its posterior (caudal) end, 

 the opposite extremity is its anterior (cephalic) end. The ce- 



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