G. C. Davhnpokt. — Primitive Streak. 



PLATE III. 



All figures are magnified 39 diameters, except Figures 12 and 12', which are magnified 



10 diameters. 



Fig. II. Chrysemys picta, Aax%2X v\tw . 



The horns of the notochordal canal are directed ptosteriad. A slight 

 pit is discernible at the anterior third of the shield. 

 Figure 11'. Ventral view of the same, showing narrow opening of the noto- 

 chordal canal. 

 Fig. 12. Ventral view of an embryo of Chelydra serpentina. 

 Fig. 12'. Dorsal view of the same. 



The medullary folds are beginning to close. The notochordal 

 opening or neurenteric canal is reduced to a small horseshoe-shaped 

 opening near the posterior end of the embryo. 

 Fig. 13. Chelydra serpentina, dorsal view. 



The horns of the notochordal opening are bent far posteriad. 

 Fig- I3'- Ventral view of the same. 



The notochordal canal opens ventrally. 



