Primitive Streak and Dorsal Notochordal Opening. 33 



third section behind the dorsal notochordal opening of this em- 

 bryo. The dorsal lip of the crescentic blastopore is cut nearer 

 the median plane on the left side than on the right, so that it over- 

 hangs the primitive streak region more on that side than on the 

 right. On both sides the lip of ectoderm is much thickened. 

 Between these lateral ectodermal elevations is situated a third, 

 the region called by Will and Mitsukuri the plug. I can distin- 

 guish no boundary between the cells which constitute the surface 

 of the so called plug and the cells of the ectoderm lateral to the 

 plug. The surface of the plug is divided in the median line by 

 a distinct groove, which begins at the posterior lip of the blasto- 

 poric opening, and extends over the plug and along the streak for 

 twenty-two sections. Figure 49 represents the tenth section be- 

 hind the open blastopore ; it passes through the posterior end of 

 the plug. The cells at the surface of the plug show a tendency to 

 become columnar, but this layer is indissolubly fused with the 

 entoderm below, while the groove is still very distinct. The 

 groove continues on backward over the streak, but becomes fainter 

 and fainter, until finally at the twenty-third section it disappears. 



b. OsotJicca odorata. — The conditions which I have described 

 for Chelydra are not peculiar to that genus. As another example 

 of a grooved plug, I will describe the condition in a single case of 

 Ozotheca odorata. A dorsal surface view (Plate II. Fig. 9) pre- 

 sents an anomalous condition of the blastoporic opening. The 

 notochordal canal is interrupted at the anterior end of the pos- 

 terior third of the shield by a thickening of cells through which 

 the neurenteric canal runs perpendicularly to open below. This 

 lumen appears in two transverse sections only, while the thickening 

 itself is fused with the ectoderm for the distance of five sections 

 anterior to the neurenteric canal. As seen in dorsal view, two 

 pairs of horns extend anteriad from this canal, and one pair pos- 

 teriad. The plug region, which is included between the two 

 posterior horns, is grooved. Figure 54 (Plate XI.) represents an 

 oblique section through the neurenteric canal. Figure 55 repre- 

 sents the third, and Figure 56 the sixth, section behind the canal. 

 On surface view and in sections one horn of the open blastopore, 

 the right, is seen to extend farther posteriad than the other. In 

 cross section the surface of the plug shows a median groove, which 

 continues backward along the streak. Figure 58 is a more highly 

 magnified representation of the plug region of the section seen in 



3 



