GERM LAYERS. 



61 



cleavage and apparently corresponds to the " Erganzungsplatte " or " com- 

 pletion plate" described in lower forms (p. 56; see also Figs. 39 and 42). 

 The germ disk continues to spread out over the yolk and at the same time the 

 area of liquifying yolk increases. The portion of the disk above the liquified 

 yolk appears translucent on surface view and is known as the area pellucida; 

 the more peripheral part of the disk is less transparent, being more closely 

 attached to the unchanged yolk, and is known as the area opaca. 



Area opaca 



Hensen's node 



Primitive streak 



Area pellucida 

 "Completion plate' 

 Head process 



Primitive groove 



Post. Hp of 

 blastopore 



FIG. 46. Surface view of embryonic disk of chick. Bonnet. 



There next appears in front of the crescentic groove and extending from its 

 middle point forward in the medial line, a linear opacity which is known as the 

 primitive streak (Fig. 46). This ends anteriorly in a knob-like expansion 

 Hensen's node. According to Duval, Hertwig, Bonnet and others, the primi- 

 tive streak is formed in the following manner. A notch or indentation appears 

 in the anterior lip of the transverse blastoporic slit (Figs. 43 and 47, A). As 



Area opaca 

 t Area pellucida 



Primitive streak 



Area pellucida 

 Area opaca 

 Primitive streak 



Blastopore 

 (crescentic groove) 



FIG. 47. Surface views of blastoderms of Haliplana, showing formation of primitive streak. 



Schauinsland. 



the germ disk is constantly spreading in all directions, if the apex of this notch 

 remains fixed, the extension of the disk posteriorly must result in a drawing out 

 of the notch into a longitudinal slit (Fig. 47, B). In other words, the horns of 

 the crescentic slit are pushed together to form a longitudinal slit. And as the 

 two lips of the slit come together they fuse, and the line of fusion is marked by a 

 shallow groove, the primitive groove. At the anterior end of the slit in the region 



