FROM LAYING TO FORMATION OF FIRST SOMITE 71 



the posterior half or two fifths of the circular pellucid area (Fig. 

 35 B). It is relatively narrow in front and widens posteriorly, 

 where it is at the same time less dense. Its anterior end usually 

 does not quite reach the center of the pellucid area. It rapidly 

 increases in length; the anterior end appears to be practically a 

 fixed point, and growth takes place posteriorly probably not by 

 addition, but between the two ends. The posterior half of the 

 pellucid area elongates simultaneously, keeping pace with the 



Fig. 35. — Surface views of two stages of the blastoderm of the egg of 

 the sparrow. (After Schauinsland.) 



A. Before the appearance of the primitive streak. 



B. The first appearance of the primitive streak. 



a. o., Area opaca. a. p., Area pellucida. Ent. Th., Thickening of en- 

 toderm, pr. str., Primitive streak. 



primitive streak which lies entirely within it in the chick and 

 most other birds. Thus the area pellucida becomes oval, then 

 pear-shaped, and the primitive streak bisects the greater part of 

 its length (Figs. 35, 36, 44, etc.). 



According to Keller the primitive streak takes its origin from a 

 crescentic area at the posterior margin of the pellucid area, which he 

 terms the sickle. The primitive streak appears as a process extending 

 forward from the center of the sickle, and, as it grows forward, the 

 lateral horns of the sickle are gradually taken into its posterior end. 

 Roller's observations and interpretations have not, however, been con- 

 firmed by subsequent investigators and they would appear to rest on 

 rather exceptional and inessential conditions. 



