74 TEXT-BOOK OF EMBRYOLOGY. 



A When incubation commences certain changes in the appearance of the 

 Wastoderm can be seen on the surface.'Lpurmg the first day a narrow band, 

 which is slightly more opaque than the Surrounding area, appears in front of 

 the closed blastopore and extends forward more than half way across the 

 area pellucida. It seems to grow from the blastopore; as a matter of fact, 

 however, the blastopore recedes and leaves the band in its trail. This is the 

 primitive streak (Figs. 44 and 45). While the streak grows the area pellucida 

 elongates in the same direction and becomes oval, the broader end being 

 anterior. Then a transparent line appears along the center of the streak 

 and terminates in front in a slight enlargement. In front of this enlargement 

 the streak is a little more opaque than elsewhere. The transparent line 

 indicates the primitive groove, which is flanked by the primitive folds, and its 



Area opaca 

 Area pellucida- 



M^^^Br 



Head process 

 Medullary folds 



Hensen's node" 

 Primitive streak 



FIG. 46. Surface view of chick blastoderm. Bonnet. 



broadened terminus is the primitive pit; the denser portion of the streak in 

 front of the pit is the primitive knot (Hensen's knot). Following the 

 development of the primitive streak there appears in front of it a narrow 

 band, less conspicuous than the streak but continuous with and extending 

 forward from the primitive knot. This is known as the primitive axis or 

 head process (Fig. 46). During these changes in appearance the blastoderm 

 also increases in total area. 



The primitive streak and the structures associated with it can be inter- 

 preted properly only in terms of sections. A transverse section through 

 the streak near its center shows both ectoderm and entoderm merged with an 

 intermediate layer which is obviously mesoderm (Fig. 47, A ) . It is the thick- 

 ness of mass resulting from the fusion of the three layers which gives the 

 opaque appearance of the primitive streak when seen from the surface. 

 The primitive groove is a linear depression in the dorsal side of the streak, and 



