74 THE DEVELOPMENT OF THE CHICK 
The later history of the primitive streak is illustrated in Figs. 
44, 51, 61, 65, ete.: 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 embryo 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 réle in the development. 
aD oe ae pire: Be 
e ee ee 
B \epeer oss Bape rae 
es EWR; 
tS) oe 
‘Fo 
a ote ni aatae ae 
~ery a = ae 5 a Saabs _ et an 
Fic. 37. — Three sections through the pene 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). 
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 which 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 shghtly 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-sae entoderm of the area opaca. The second 
