CHAPTER VI 



FROM THE PRIMITIVE STREAK STAGE TO THE 

 APPEARANCE OF THE SOMITES 



THE PRIMITIVE STREAK AS A CENTER OF GROWTH; THE GROWTH 



OF THE ENTODERM AND THE ESTABLISHMENT OF THE 

 PRIMITIVE GUT; THE GROWTH AND DIFFERENTIATION OF THE 

 MESODERM; THE FORMATION OF THE NOTOCHORD; THE 

 FORMATION OF THE NEURAL PLATE; THE DIFFERENTIATION 

 OF THE EMBRYONAL AREA. 



The Primitive Streak as a Center of Growth. The impor- 

 tance of the primitive streak embryologically, is due chiefly to the 

 way it is involved in the establishment of the germ layers. 

 Representing as it does the fused lips of the blastopore it marks 

 the location of entoderm invagination. The mesoderm also 

 arises at the primitive streak region. The general appearance 

 and the location of the primitive streak are both well shown in 

 embryos of 16 hours of incubation (Fig. 8). In embryos which 

 have been incubated 18 hours (Fig. n) the primitive streak is 

 still the most conspicuous feature. Structurally it is little 

 changed from the conditions seen in 1 6-hour chicks, but it appears 

 to be somewhat more caudally located. In 21 to 22-hour em- 

 bryos (Fig. 14) the primitive streak lies still farther caudal in 

 the blastoderm. Its change in position is relative rather than 

 actual. The apparent change in the position of the primitive 

 streak is due to the fact that growth is taking place more rapidly 

 cephalic to it than caudal to it. This tendency is in evidence 

 throughout the early growth of the embryo. The cephalic 

 region is precocious in development. As development pro- 

 gresses we shall find the primitive streak occupying a constantly 

 more posterior position in the body and being more and more 

 overshadowed by the greater growth of the structures lying 

 cephalic to it. 



The structure of the primitive streak region is best shown 

 by transverse sections. In the sections diagrammed in Figure 



3 33 



