82 



THE DEVELOPMENT OF THE CHICK 



entoderm for a considerable period after the formation of the head- 

 fold. A longitudinal section shows the head-process as an append- 

 age to the anterior end of the primitive streak, or the primitive 

 knot (Fig. 42). 



m.n. 



Fig. 43. — Diagrams to illustrate the theory of concrescence as applied to 

 the primitive streak of the bird. The central area bounded by the broken 

 line represents the pellucid area; external to this is the area opaca, showing 

 as concentric zones the germ-wall (G. W.), the zone of junction (Z. J.), 

 and the margin of overgrowth (M. O.). m. n., Marginal notch. For de- 

 scription see text. 



The most obvious interpretation of the head-process is as 

 an outgrowth from the primitive knot. But another, and more 

 probable interpretation in view of all the facts, is that the head- 

 process is a later stage of the anterior end of the primitive streak; 



