G. C. Davenport. — Primitive Streak. 



PLATE 11. 



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 serpentbia. 



The horns of the notochordal canal of embryo /3 point anteriad. The 

 anterior lip of the notochordal opening is distinctly lobed, i. e. divided 

 at the axial point by a median notch. Embryo ^ is growing forward 

 for the greater portion of its length underneath embryo a. The axis 

 of a lies anterior and oblique to the axis of /3. A notochordal opening 

 transverse to the axis, such as exists in 3, is not discernible on a. At 

 the point of union of a and /3, 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. Chelydra 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 Chelydra serpentina^ dorsal view. » 



The axis of individual /3 is almost perpendicular to that of individ- 

 ual a. The horns of the dorsal notochordal opening of /3 are directed 

 anteriad. 



Note. — By an oversight, the lettering of these embryos has been inter- 

 changed \n 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, con- 

 tinuous with the groove on the shield, is to be followed to the left 

 border of the fissure, where a break prevents its being traced further. 

 Fig. 9. Ozotheca odoiuita, dorsal view. , 



The outline of the notochordal opening is irr gular. One pair of 1 

 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. Chelydra serpentina, dorsal view. j 



The dorsal opening of the notochordal canal is a nearly transverse 

 slit. A median notch exists on the anterior lip. 

 Fig. 10'. Ventral view of the same, showing the ventral opening of the noto- I 

 chordal canal. 



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