8 Primitive Streak and NotocJwrdal Canal in Chelonia. 



of embryo a, we find that this distinct entodermic layer thins out 

 and eventually disappears, so that the floor of the canal is com- 

 posed of one layer only (Figs. 15 and 16), 



2. Chrysemys picta and Ozotheca odorata. — A series of sections 

 through the shield of an embryo of the former genus shows the 

 same peculiarity in regard to the entoderm. Figures i and i' 

 represent dorsal and ventral surface views respectively of this 

 embryo. From the ventral view we see that the lower wall of 

 the notochordal canal is breaking away. The breaking throngh is 

 not yet completed, however, for anterior to the existing open- 

 ing a portion of the floor still remains. Posterior to the opening 

 a considerable portion of the primitive knob also remains intact. 

 From the region of the knob I have drawn two sections. Figure 

 45 (Plate IX.) is taken from the anterior region of the unbroken 

 knob; Figure 46 falls just anterior to the dorsal lip of the open 

 blastopore. In Figure 45 the ventral floor of the notochordal 

 canal is composed of a layer of cells about three rows thick. For 

 a short space at the extreme right, however, a few cells mingled 

 with yolk globules, and indicated by an asterisk, are separated off 

 by a distinct line as an independent layer. According to Will this 

 is the region of the developing secondary entoderm. Mitsukuri 

 ('93, pp. 256, 257), on the other hand, if I correctly understand 

 his idea and his application of Hubrecht's ('90, p. 522) theory of 

 precocious segregation, would be compelled to say that in this in- 

 stance palingenetic and ccenogenetic hypoblast have failed to fuse. 

 As we proceed backward we find that with each succeeding section 

 this lateral area increases in its extent toward the axial line, and 

 finally (Fig. 46,*) is continuous with a similar area, developed 

 a little farther back on the opposite side, forming a complete and 

 distinct layer next the yolk. 



A lack of sections of earlier stages compels me to leave un- 

 touched the question concerning primary and secondary hypo- 

 blast. I may, however, state that in two instances I have found 

 that the free edges of the secondary entoderm stood out along the 

 lateral margins of the streak behind the blastopore in such a way 

 as to give one the impression that this secondary entoderm is 

 growing in from the sides underneath the streak in the manner 

 described by Will for the ectoderm on the dorsal surface of the 

 streak. I believe, however, that the secondary entoderm is not 

 developing in this region, but is in process of dissolution. My 



