74 THE DEVELOPMENT OF THE CHICK 



The later history of the primitive streak is illustrated in Figs. 

 44, ol, 61, 65, etc.: the embryo arises in front of it around the 

 head-process as a center; the anterior end of the primitive streak 

 marks the hind end of the differentiated portion of the embryo. 

 As the embryo grows in length the primitive streak decreases (cf. 

 measurements in table), until finally, when the completion of the 

 embryo is indicated by the formation of the tail-fold, the primi- 

 tive streak disappears. The primitive knot and primitive pit 

 occupy its anterior end at all stages, and, as the embr3"o differen- 

 tiates from the anterior end of the primitive streak, the primitive 

 pit must be regarded as moving back along the line of the primi- 

 tive groove, always representing its anterior end. 



Sections. The preceding sketch of the superficial appearance 

 of the primitive streak must now be followed by a careful exami- 

 nation of its structure and role in the development. 



c 



Fig. 37. — Three sections through the primitive streak of a sparrow at a 

 stage intermediate between Figs. 35 and 36. x 230. (After Schauinsland.) 



A. In front of the primitive streak. 



B. Through the anterior end of the primitive streak (primitive knot). 



C. About through the center of the primitive streak. 



All recent authors are agreed that the primitive streak owes 

 its origin to a linear thickening of the ectoderm, from Avhich cells 

 are proliferated between the ectoderm and the entoderm, forming 

 a third layer, the mesoderm. Figs. 37 A, B, C show three trans- 

 verse sections through a blastoderm of the sparrow slightly more 

 advanced than the stage shown in Fig. 35 B. The first section 

 is just in front of the primitive streak. The ectoderm is thick 

 in the center and thins gradually toward the margin of the area 

 pellucida, becoming decidedly thin in the region of the area opaca. 

 The thin entoderm of the area pellucida unites peripherally with 

 the thick yolk-sac entoderm of the area opaca. The second 



