Primitive Streak and Dorsal Notochordal Opening. 23 



eiidrm. d. /3 and etidrm. v. 13. The layer of cells marked en'drui. a 

 comprises a portion of the entoderm of a, which is fused with that 

 of /3 at the posterior end of the streak at a point in the section 

 marked by an asterisk. 



A second example of a double embryo was found two weeks 

 later than the one just described. As in the preceding case, so 

 in this one, we will designate the two individuals as a and /S. The 

 embryo with an elongated streak posterior to the blastopore (Plate 

 II. Fig. 8, left half) will be distinguished as a. (By an error a and 

 /3 are interchanged in Fig. 8. See Diagram I., p. 6, for correct 

 lettering.) This streak, which is so much elongated as to resemble 

 the handle of a dipper, the bowl of which is represented by the 

 second embryo, /S, furnishes a striking parallel to the condition of 

 the streak in the emu figured by Haswell ('87) in his Plate VIII., 

 and again in his Diagrams 5 and 6 (p. 584), which " are intended 

 to illustrate the manner in which, as pointed out by Duval, the 

 anterior end of the primitive streak comes in its later stages to be 

 situated so far forwards simply by the considerable extension of 

 the area pellucida on all sides." 



In the turtle (individual a) the streak is seen to be divided at its 

 anterior third by a sort of ridge. The portion anterior to the ridge 

 is comparable in extent to the streak in the emu described by 

 Haswell, and possesses a true primitive groove, shown in Figure 8 

 as a fine sharp line, whereas the posterior two thirds show along 

 the axis a rather broad depression instead of a true groove. Sec- 

 tions show, however, that these two grooves are structurally con- 

 tinuous with each other, and that therefore the posterior portion is 

 not due simply to an accidental folding of the blastoderm induced 

 during the process of hardening. Unfortunately, the whole blasto- 

 derm was not preserved ; consequently the condition of its edge 

 behind the elongated streak of a cannot be stated ; it is uncertain 

 whether this streak extended to the edge of the blastoderm and to 

 a marginal notch, or even whether a marginal notch existed. The 

 margin of the shield of embryo a extends around to, and is con- 

 tinuous with, the blastopore of embryo /3. One horn of the blasto- 

 pore of embryo /S (Plate II. Fig. 8) was unobserved under the low 

 magnification at which this figure was drawn. Diagram I. has 

 been reconstructed from the sections in order to show the position 

 of this horn of the blastopore, and the extent of the notochordal 



