Primitive Streak and Dorsal Notochordal Opening. 25 



terminate ventrally in a seam or closed fold, which dips down into 

 the mesoderm. The presence of this seam is indicated in section 

 by the fact that the boundaries of the cells here stain more deeply 

 than mere cell walls, and that the cells which adjoin it are more 

 or less flattened perpendicular to the plane of the fold. A single 

 cell on the left bounding the seam and reaching to the surface is 

 particularly remarkable in this respect. Its dorso-ventral length 

 corresponds with that of two and a half somewhat elongated cells 

 on the opposite side of the seam. Even the nucleus of this 

 greatly flattened cell is correspondingly altered in form. This cell 

 contains, besides the nucleus, two yolk globules. This condition 

 of the groove occurs, as already stated, on the floor of the canal 

 in the seventh section anterior to the dorsal lip of the blastoporic 

 opening. The groove, however, continues anteriad for four or five 

 sections more before it fades away. If we follow the groove in 

 sections posterior to the one described, we find that it is distin- 

 guishable in all the sections in the median line of the floor of the 

 invagination; as we pass backwards, it increases in distinctness, 

 and becomes continuous with the primitive groove occupying the 

 axis of the streak behind the blastopore. 



The next figure (Plate IV. Fig. 18) represents the fourth section 

 behind the blastopore, and passes through the region of the so 

 called plug of Will, Mehnert, and Mitsukuri (compare also Plate 

 V. Fig. 23). Mesoderm and entoderm are still distinguishable as 

 separate layers, but ectoderm and mesoderm are fused. Along 

 the margins or lateral boundaries of the streak I can distinguish in 

 sections no such separation into ectoderm and entoderm as Will 

 has described for both the gecko and the turtle. Will criticises 

 Mitsukuri's drawings for their'failure to bring out this separation. 

 I have repeatedly and carefully gone over all my sections in order 

 to discover such a line of demarcation. Among twenty-six embryos 

 of about the same stages as those described by Will, I have 

 found but once and in only one section — then on only one side 

 of the streak — evidence of such a separation. I consider this 

 separation to be mechanical, for I cannot find a trace of it in the 

 preceding or succeeding sections. 



Figure 23 (Plate V.) represents under greater magnification the 

 fused area or streak shown in Figure 18 (Plate IV.). Here also 

 the outline of each cell and nucleus was carefully made by aid 

 of the camera lucida. At the extreme right of the figure are 



