20 Primitive Streak and Notochordal Canal in Chelonia. 



and comes almost to reach the axis of a (Plate VI, Fig. 27). This 

 upward pressure exerted by /3 upon one side of a Hfts one half of 

 a above the level of the other half, so that the ectoderm fails to 

 fuse in the line of the medullary groove (Plate V. Figs. 24-26). 



This division of the ectoderm in the axial line reminds one so 

 strongly of the ectodermal spina bifida produced by Kollmann 

 ('93> PP- I35» 136) in the chick and duck by means of overheating, 

 and of the many grades of spina bifida induced by Hertwig ('92) 

 in the frog, — a spina bifida which also ofttimes healed later, — 

 that one has little hesitancy in considering this to be another case 

 of partial spina bifida. The ectoderm remains separated in the ax- 

 ial line for a distance of sixty-one sections. At the anterior end of 

 the embryo, however, the ectodermal halves again fuse. Hence we 

 find that at the far anterior and posterior ends of a, — the points 

 where the pressure exerted by /3 is least, — the ectoderm is not 

 severed. In the region of non-fusion I cannot detect the slightest 

 evidence of mechanical rupture. Each ectodermal half is smoothly 

 rounded off at its axial edge (Plate V. Fig. 24) without the slightest 

 evidence of a former union. In section forty-one (Plate V. Fig. 25) 

 there is decided evidence of fusion, not however between the 

 ectoderm of the two sides, but between the ectoderm and entoderm 

 of the right side. Upon comparing Figure 26 (Plate V.) with 

 Figure 27 (Plate VI.) it appears that the entoderm (^en'drm. d. /3) 

 lying immediately beneath the ectoderm on the left hand side of 

 a does not belong to a at all, but is that portion of the entoderm 

 of /8 which has pushed forward underneath a. What then has 

 become of one half of the entoderm of a } If we compare Fig- 

 ures 24, 25, 26 (Plate v.), and 27 (Plate VI.), it will be seen 

 that this portion fused with the notochordal entoderm of /3 

 {cndnn. d. yS), and that it probably broke away together with the 

 ventral entoderm of /3 {endrm. v. /3) when the notochordal canal 

 of that individual became continuous with the subgerminal space. 

 Perhaps the entoderm of a, like the ectpderm, was bifid in the 

 axial line in those seven sections in which it has disappeared on 

 the left side only; for we find that, when the entoderm was 

 continuous in the axial line, as in Plate V. Fig. 26, the rupture 

 occurred lateral to the axis. The section which is shown in 

 Figure 27 (Plate VI.) falls near the dorsal notochordal opening 

 of /3 and towards the anterior end of a. The ectodermal and 

 likewise the entodermal halves of a are now continuous. The 



