Regression of the Neurenteric Canal along the Streak. 43 



the anterior or dorsal lips of the blastopore are approaching each 

 other to fuse in the axial line of the embryo. (See Plate IV. Fig. 

 17, Plate V. Fig. 23, and Plate X. Fig. 52.) In a few series it is 

 possible in the sections to trace the groove into this region, but in 

 those instances in which the streak becomes depressed the groove 

 is usually obliterated. 



In Diagram IV. the ectoderm has made much progress in its 

 journey laterad and dorsad. For the sake of clearness, I have 

 represented this movement as occurring entirely in one plane. As 

 a matter of fact, in consequence of the obliquity of the notochordal 

 canal in early stages, caused by the uneven pace of the two lips in 

 opening and closing, we must conceive of a migration not only 

 laterad and dorsad, but also posteriad ; for the ectoderm which 

 adjoins the notochordal area in Diagram IV. will not only fuse in 

 the dorsal lip at a point above, but in a plane posterior to, the one 

 in which it now lies. When the canal comes in later stages to 

 occupy a more perpendicular position, when, in other words, the 

 ventral lip comes to open so rapidly that it has almost caught up 

 with the dorsal lip in its process of closing, then the migration will 

 be accomplished more nearly in the dorso-ventral plane, as is 

 illustrated in my diagrams. 



Mitsukuri's ('86) Figure 10 (Plate III.) represents a stage inter- 

 mediate between those figured in my Diagrams IV. and III. The 

 dorsal lip in Mitsukuri's figure is almost closed, while the ventral 

 floor remains intact. In my Diagram III. the dorsal lip has fused 

 in the axial line, but a differentiation into ectoderm and notochord 

 has not yet been accomplished here. The condition figured in 

 this diagram is illustrated in the sections shown in Figures 22 

 (Plate V.) and 46 (Plate IX.). 



Mitsukuri's ('86) Figure 19 (Plate IV.) represents a stage again 

 intermediate, so far as the differentiation of the notochord is con- 

 cerned, between my Diagrams III. and II. In Mitsukuri's figure 

 the ectoderm and notochord are differentiated as distinct layers, 

 save for a very short space at the axis. The ectoderm and noto- 

 chordal entoderm may become completely distinct from each 

 other, as in Diagram II., for a space of several sections before the 

 separation and disintegration of the entoderm is accomplished. 

 Mehnert's ('92) Figure 24 (Taf. XIX.), or Mitsukuri and Ishikawa's 

 ('86) Figure 11 (Plate III.), as well as my own sections, illustrate 

 such a condition. 



