26 Primitive Streak and Notochordal Canal in Chelonia. 



represented the first two ectoderm cells, which show a distinctly 

 columnar form. Adjoining these on the side toward the primitive 

 groove are four ectoderm cells which are distinctly separated from 

 the lower lying cells. The last of the four is the one indicated by 

 the dotted line running from the letters ecdrm. These four cells, 

 however, are not of columnar form. The next cell which adjoins 

 these four, again on the side toward the groove, is exactly like 

 them in character, but is not separated from the cell layer below. 

 The two succeeding cells, continuing in the same direction, are 

 dividing along a plane parallel to the ectodermal surface, and are 

 undoubtedly contributing to the formation of the deeper cell layer. 

 In the centre of the streak a V-shaped groove exists, from the 

 bottom of which a sharp line extends still deeper, as though mark- 

 ing the direction of a fold. This groove continues along the axis 

 of the streak for the space of seven sections behind the open blas- 

 topore. In Figure 19 the groove is no longer evident. The ecto- 

 derm remains fused with the mesoderm in this and four succeeding 

 sections, yet it is discernible as a well marked layer continuously 

 across the region of the streak, as Mitsukuri and Ishikawa ('86, 

 p. 30) have described it in Tryonix. There is not the slightest 

 indication in the grooved region, or behind it, of that differentia- 

 tion of the streak into " Mittelfeld " and " Randfeld " described 

 by Will for the gecko ('92'', Plate II.) and for the turtle ('93, pp. 

 569—571). In Figure 19 it is shown that the mesoderm and ento- 

 derm are fused. From this section posteriad the entodermic layer 

 begins to grow thinner, until, after a long stretch, a condition is 

 reached in which there is below the ectoderm, and entirely free 

 from it, a series of clumps and strands of cells scattered on the 

 yolk or partially embedded in it. This condition extends for 

 some distance laterad to the axis of the streak on either side 

 (Plate V. Fig. 20). This stretch of cells is really narrower in 

 extent than the mesoderm of the streak in Figure 19, for Figure 

 20 is outlined under a higher magnification than the preceding 

 figure. 



We now pass along the streak until we reach the condition 

 seen in Figure 21 (Plate V.). In this section the ectoderm seems 

 to be fused in the median line with a lower lying continuous row of 

 cells, which stretches laterad for a short space on either side of the 

 region of fusion. This lower layer shows a tendency to fuse here 

 and there all along its extent with the ectoderm. Unfortunately 



