EAEE FORM OF THE BLASTODERM OF THE CHICK. 877 



The following topographical measurements were taken from 

 the mounted preparation by the aid of the camera lucida : 

 Blastoderm 9 12 mm. long, 8*04 mm. wide. 

 Marginal notch (" Randkerbe," Rauber) "46 mm. deep. 

 Area pellucida 2*4 mm. long, 116 mm. wide. 



Distance from the anterior end of the area pel. to the corresponding 



margin of the blastoderm 3"56mm. 

 „ posterior „ „ ,, 316 „ 



„ right side „ „ „ 2-8 „ 



„ ■ left side „ „ „ 3-48 „ 



Primitive streak 1"04 mm. long; the distance between its 

 fore end and the anterior border of the area pellucida 1'2 mm. 

 Distance of the marginal notch from the posterior end of the 

 primitive streak 2*2 mm. 



The blastoderm is subcircular, and the marginal notch is 

 situated on the right side instead of at the hind end of the 

 main axis.^ The area pellucida has an irregular pyriform out- 

 line, with the smaller end directed backward. It is placed a 

 little excentrically, being a little nearer the right side than the 

 left. 



In the pellucid area two regions are easily distinguished ; 

 one, which is sometimes called the embryonic shield (s), is 

 pyriform, occupies the central and rear portion of the field, 

 and bears the primitive streak; the other is less opaque, peri- 

 pheral in position, and crescentic in shape. The horns of the 

 crescent are not quite symmetrical, the left being the longer, so 

 that the hind end of the shield lies nearer the right than the 

 left side of the pellucid area. The shield shades off very 

 gradually into the clearer crescentic region. 



The primitive streak is very well marked, and shows a well- 

 defined primitive groove. In front of the primitive groove a 

 linear opaque area is seen to vanish a liltle before the front 

 margin of the shield, and appears to be a continuation of 

 the primitive streak itself. This linear area is the " kopfort- 

 satz" of KoUiker (" Axenplatte,'' Kupffer), and represents, 



' The main axis passes to the left of the primitive streak, but parallel 

 with it. 



