III.] THE PRIMITIVE STREAK. 53 



latter structure, and its relation to the embryo, can how- 

 ever only be understood by comparison with the develop- 

 ment of the lower forms of vertebrate life. 



It will be remembered that in surface views of the 

 unincubated blastoderm a small arc, at what we stated 

 to be the posterior end, close to the junction between 

 the area opaca and the area pellucida is distinguished 

 by its more opaque appearance. In the surface view 

 the primitive streak appears as a linear opacity, which 

 gradually grows forwards from the middle of this arc 

 till it reaches about one-third of the diameter of the 



FIG. 14. 



pr s 



AREA PELLUCIDA OF A VERY YOUNG BLASTODERM OF A CHICK, 

 SHEWING THE PRIMITIVE STREAK SHORTLY AFTER ITS 

 FIRST APPEARANCE. 

 pr.s. primitive streak ; ap. area pellucida ; a.op. area opaca. 



area pellucida. During the formation of the primitive 

 streak the embryonic shield grows fainter and finally 

 vanishes. When definitely established the primitive 

 streak has the appearance diagrammatically represented 

 in Fig. 14. 



