File:The primitive streak and notochordal canal in Chelonia (Plate II) BHL4780734.jpg

PLATE II. All figures magnified 39 diameters, except Figures 6 and 8, which are magnified 10 diameters.
Fig. 5. Dorsal view of a double embryo of Chelydra serpentina. The horns of the notochordal canal of embryo 8 point anteriad. The anterior lip of the notochordal opening is clistinctly lobed, i. e. divided at the axial point by a median notch. Embryo β is growing forward for the greater portion of its Jength underneath embryo a. The axis of a lies anterior and oblique to the axis of β. A notochordal opening transverse to the axis, such as exists in β is not discernible on a. At the point of union of a and β the shields of both individuals are thrown more or less into folds.
Fig. 6. Chelydra serpentina, dorsal view. The head fold and medullary folds are formed. The notochordal opening has assumed a distinct horseshoe shape, whose concavity is directed posteriad. The external opening of the neurenteric canal is much reduced in size.
Fig. 7. Chelvdra serpentina, dorsal view. The dorsal notochordal opening is an almost transverse slit. The horns, however, have begun to bend slightly posteriad. A groove reaches from the posterior lip of the dorsal opening to the posterior edge of the shield.
Fig. 7'. Ventral view of the same. The notochordal canal has broken through below.
Fig. 8. Double embryo of Chelvdra serpentina, dorsal view. The axis of individual β is almost perpendicular to that of individual a. The horns of the dorsal notochordal opening of β are directed anteriad.
Note. — By an oversight, the lettering of these embryos has been interchanged in Figure 8. (See also Diagram I. in the text, page 6.)
A long grooved streak leads posteriad from the notochordal opening of a. Beyond the limits of the shield of a a groove or depression, continuous with the groove on the shield, is to be followed to the left border of the figure, where a break prevents its being traced further.
Fig. 9. Osotheca odorata, dorsal view. The outline of the netochordal opening is irregular. One pair of horns points posteriad, and two pairs anteriad.
Fig. 9'. Ventral view the same. The anterior end of the notochordal canal has opened below, but seems to be closed again by a clump of tissue through which a narrow canal sinks perpendicularly from the dorsal to the ventral surface.
Fig. 10. Chelvdra serpentina, dorsal view.‘The dorsal opening of the notochordal canal is a nearly transverse slit. A median notch exists on the anterior lip.