FROM LAYING TO FORMATION OF FIRST SOMITE rip 
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 simultancously, keeping pace with the 
Fic. 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 Koller the primitive streak takes its origin from a 
erescentic 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. 
Koller’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. 
