G. C. Davenport. — Primitive Streak. 



PLATE VI. 



All figures magnified 73 diameters, except Figures 30 and 31, which are magnified 



240 diameters. 



Fig. 27. Section indicated by the line 27-27, Fig. 5. The ectoderm of a has 

 fused in the axis. The entoderm of a (eti'drm. a) has fused with 

 the entoderm of /3 {eti'drm. v. ^) at the point marked by an 

 asterisk. The notochordal canal of ^ is open at its anterior end. 



Fig. 28. Sagittal section through the remaining portion of j3, indicated by the 

 short line in the plane 28, Fig. 5. 



Fig. 29. Section lateral to Fig. 28, cut in the direction of line 29, Fig. 5. The 

 point of union between individuals a and ^ is seen midway between 

 the letters a and ^. 



Fig. 30. Transverse section across the axis of embryo a in the region of line 

 30-30, Fig. 5. The ectoderm and entoderm are fused in the axis. 

 A small canal, can. n'ent. a, sinks perpendicularly into this region 

 of fusion. 



Fig. 31. Transverse section, cut along the line 31-31, Fig. 5, across embryo a. 

 The ectoderm and entoderm are separated from each other at the 

 axis, but both are grooved or constricted at that point. 



