28 Primitive Streak and Notochordal Canal in Chelonia. 



area in this figure, as well as in Figure 39, it is impossible to 

 distinguish ectoderm from entoderm, excepting at the margins 

 through which the yolk has streamed out. Wherever the ecto- 

 derm of opposite sides of the median plane has succeeded in 

 uniting in this central area, a differentiation into ectoderm and 

 entoderm is discernible (Figs. 38 and 39). Seven sections behind 

 that shown in Figure 40 (Fig. 41) the edges of the ectoderm fail 

 by a considerable distance to meet in the mid-line. At the margins 

 of the uncovered area the ectoderm bends downward and lateral- 

 ward to become continuous with the cells of the entoderm and 

 yolk. This central area may be considered to be one of uncovered 

 yolk. Figure 42 is a drawing under a higher magnification of this 

 uncovered area and the adjacent covered area on one side of it, 

 twelve sections behind that of Figure 41. This figure was out- 

 lined with the camera lucida even to the smallest details. In this 

 figure the layer which is folded in under the ectoderm shows no 

 tendency to fuse with the ectoderm above it. In other sections, 

 however, this whole lateral region is composed of a mass of cells 

 upon whose dorsal surface the ectoderm is not well differentiated. 

 The margins of the ectoderm continue to be separated in the axial 

 line for the space of twenty-five sections. Wherever the yolk has 

 streamed out over the dorsal surface of the ectoderm, the dorsal 

 ectodermal surface seems to be absorbing this yolk, for in many 

 sections the external surface of the ectodermal layer has no 

 distinct boundary, and a few nuclei are present in the yolk 

 above it. 



As we pass po.steriad the separated ectodermal margins gradu- 

 ally approach each other, and finally fuse (Fig. 43). In the 

 immediate vicinity of the point of fusion the ectoderm is not 

 separated from the entoderm below. From this point of union 

 posteriad the entoderm begins to thin out, and finally, when we 

 arrive at the sixty-fourth section behind the posterior end of the 

 uncovered area, we meet such a condition of ectoderm and scat- 

 tered entoderm as exists over the whole area opaca. The cellular 

 condition of this streak within the area opaca behind the un- 

 covered area is similar to the condition found in the streak within 

 the area pellucida of embryo a (Plate II. Fig. 8, and Plate IV. 

 Figs. 18 and 19, and area opaca, Plate V. Fig. 20) of the preceding 

 pair of twins. In embryo a (Fig. 8) the ectoderm must have 

 fused in the axial line as rapidly, or nearly as rapidly, as the 



