FORMATION OF THE PRIMITIVE STREAK 29 



A schematic interpretation of four steps in the concrescrnce 

 of the margins of the blastopore is given in the diagrams of 

 Figure 9. The blastoderm shown in surface-view plan in 

 Figure 9, A , is approximately at the same stage of gastrulation as 

 that indicated in Figure 7, B. To avoid complicating the 

 diagarm, the entoderm has not been shown in Figure 9. Num- 

 bers have been placed along the lip of the blastopore to facilitate 





marginal notch 



PIG. 9. Schematic diagrams to illustrate the concrescence theory of the origin 

 of the primitive streak. (After Lillie.) For explanation see text. 



following the changes in position undergone by the points to 

 which they are affixed. As the margins of the blastoderm 

 adjacent to the blastopore grow, they tend to converge in the 

 direction indicated by the arrows in Figure 9, B. The anterior 

 lip of the blastopore is folded on itself by this converging growth. 

 The middle point of the lip, i, comes to lie within the margin of 

 the blastoerdm, and points, 2,2, which formerly lay laterally are 



