G. C. Davenport. — Primitive Streak. 



PLATE I. 



All figures magnified 39 diameters. 



Fig. I. Dorsal view of an embryo Chrysemys picta. 



The concavity of the dorsal notochordal opening is directed posteriad. 

 Fig. i'. Ventral view of the same. 



The jagged edges of the ventral notochordal opening show that the 

 notochordal canal is in process of breaking through ventrally. 

 Fig. 2. Chelopus insculptus, dorsal view. 



The concavity of the notochordal opening is directed anteriad. 

 Fig. 2'. Ventral view of the same. 



The notochordal canal has broken through at its anterior end. 

 Fig. 3. Chrysemys picta, dorsal view. 



The notochordal opening is a nearly transverse slit. The width of 

 the broad notochordal canal is discernible on dorsal as well as ventral 

 view. 

 Fig. 3'. Ventral view of the same. 



The anterior portion of the floor of the notochordal canal has broken 

 through. 

 Fig. 4. Chelydra serpentina, dorsal view. 



The horns of the notochordal opening point posteriad. Along the 

 axis of the embryo, reaching from the posterior lip of the notochordal 

 opening to the posterior end of the shield, extends a well marked 

 groove. 

 Fig. 4'. Ventral view of the same. 



Lateral portions of the shield have bent underneath during the pro- 

 cess of hardening. The groove is discernible on the ventral surface 

 as a light (unshaded) longitudinal band at the posterior end of the 

 shield. The floor of the notochordal canal has broken through at its 

 anterior end. 



