G. C. Davenport. — Primitive Streak. 



PLATE VIII. 



All figures magnified 73 diameters, e.xcept Figures 35, 42, and 44, which are magnified 



420 diameters. 



Fig. 35. Transverse section in the region of the notochordal canal, taken at 

 the position of line 35, Fig. 44. Surface views of this embryo are 

 seen in Figs. 3, 3', Plate I. 



Figs. 36-43. Transverse sections of the streak taken at the points indicated in 

 Fig. 44 by corresponding numbers. 



Fig. 39. A layer of yolk. cl. vt., is spread out over the ectoderm. 



Fig. 40. The ectoderm is separated at the points ec'en. The yolk, cl. vt., is 

 streaming through this gap. 



Fig. 41. The ectoderm is widely separated at the points ec'en. The yolk is 

 spread out-over the ectoderm. 



Fig. 42. Cells and yolk globules outlined under higher magnification by means 

 of the camera lucida; ecen. are points at which the ectoderm is 

 proliferating nuclei ventrally and laterally. 



Fig. 43. The ectoderm is differentiated entirely across the dorsal surface, but 

 it is mixed with, and seems to be feeding on, the yolk cells within 

 and above it. 



Fig. 44. A reconstruction in the sagittal plane from cross sections. The 

 'sagittal plane falls at the points marked 44 on the transverse sec- 

 tions, Figs. 36-43. The lumen of the notochordal canal is filled for 

 the posterior third of its extent with a substance which I interpret 

 as degenerating cells. 



