CHAPTER XXX. 



THE PRIMITIVE STREAK AND ORIGIN OF THE MESODERM 



All that has been described so far has actually taken place before 

 the egg" is laid. The real beginnings of a distinguishable embryonic area 

 may be said to start with the primitive streak. While there are various 

 theories as to just how this thickened streak is formed, the most logical 

 and intelligible is that it is a thickening formed by the two lips of the 

 blastopore meeting and growing downward. 



To make this clear, the student will remember that throughout this 

 entire volume, the blastula has been considered a hollow sphere com- 

 posed of a single layer of cells, and the gastrula was this same hollow 

 sphere after it had indented so as to form two layers. The opening 

 where the indentation took place was called the blastopore. 



In the chick-embryo we are to 

 think of this blastula, however, not 

 as a sphere, but as sausage-shaped, 

 with the indentation taking place 

 from about the center of the long 

 axis to one end. Thus we do not 

 have a round blastopore, but an 

 elongated one. And it is the closing 

 of the lips along this elongated slit 

 which forms the thickening called 

 tue primitive streak (Fig. 262). It 

 is clearly setn at sixteen hours of 

 incubation, not only as a thickening, 

 but as an indentation the primitive 

 groove with ridge-like thickenings 

 flanking each side, and extending 

 from the area opaca to almost the 

 center of the blastoderm. The part lying closest to the area opaca is 

 the caudal end, and the direction of the streak forms the long axis of 

 the embryo. At the cephalic end of the primitive groove there is a deep- 

 ening called the primitive pit, and directly anterior to this the two lips 

 of the primitive folds meet in the midline to form a small rounded eleva- 

 tion known as Hensen's node. This node serves as the region of demar- 

 cation separating the fast disappearing primitive streak from the noto- 

 chord, which forms cephalad to it in the long axis of the embryonic 

 area. The growth of the embryo is much greater headward than cau- 

 dally or laterally, so that the antero-posterior axis becomes considerably 

 elongated. 



Fig 262. 



Dorsal view of 16 to 20 hour chick embryo 

 showing primitive streak, primitive groove, 

 primitive node, beginning of neural groove, 

 blood-islands, and extent of mesoderm. (After 

 Duval.) 



