24 Primitive Streak and Notochordal Canal in Chelonia. 



invaginations ; the positions of the sections figured in Plates IV. 

 and V. are indicated by parallel lines, which are numbered to 

 correspond with the numbers of the figures. 



Since these embryos, a and /8, were situated almost at right 

 angles to each other, a series of sections could be made to furnish 

 nearly sagittal sections of /3 aijd cross sections of a. The sections 

 were cut in succession from the right toward the left portions of /3. 

 A description of a sagittal section through embryo /3 (Plate IV. 

 Fig. 14) has already been given. Figure 15 (Plate IV.) represents 

 a section which passes through the horn of the blastopore of 

 embryo /3 (the depression near the letter a in Figure 15), and the 

 margin of the shield of embryo a, (at the letter /3 of Figure 15). 

 Compare Diagram I. 



A little below and to the right of the cut horn of the blasto- 

 pore of yS is seen a cavity which seems to be continuous with the 

 notochordal canal of a; it has been correspondingly lettered in 

 Figure 14. The section shown in Figure 16 (Plate IV.) passes 

 beyond the left horn of the blastopore of embryo /8, and across 

 the axis of a. The region of the notochordal canal of /S is indi- 

 cated in Figure 8 (Plate II.) by the line along the axis anterior to 

 the blastopore. This line is much too sharp, for it is impossible 

 from the sections to make out any corresponding depression of 

 the surface. The ventral wall of the notochordal canal of a is un- 

 broken (Fig. 16), and there is no division into primary and second- 

 ary entoderm discernible. The letter ^ indicates in this section, 

 too, the margin of the shield of embryo a. Figure 17 (Plate IV.) 

 represents the seventh section anterior to the dorsal lip of the 

 notochordal canal of a. We have already reached a point in this 

 embryo, in passing from in front backwards, where the ectoderm 

 and entoderm of the dorsal lip are fusing. This fusion becomes 

 more and more pronounced as we pass backward through each 

 of the six sections which lie in front of the dorsal opening of the 

 notochordal canal. The mesoderm shows a differentiation into 

 splanchnic and somatic portions, and is separated from the ento- 

 derm. In the mid-line on the floor of the canal a slight groove 

 {^sid. pr. /.) is evident, even under low magnification. A cross 

 section of this groove is shown under higher magnification in 

 Figure 22 (Plate V.). The cell walls, nuclei, and yolk granules 

 were all carefully outlined in this drawing by means of the camera 

 lucida. In both Figure 17 and Figure 22 this groove appears to 



