Comparison of gastrulation and early neurulation stages between the turtle and the chick.

Stage correspondances at turtle stage 0 to 2 were mainly based on Otx2 expression pattern, which appears highly dynamic and very similar in the turtle and in the chick (see Dicussion). Additional morphological criteria were used at turtle stages 2 and 3, respectively characterized by the presence of the notochord in its anterior-most parts (head process : stage HH5 in the chick and 2 in the turtle), and the elevation of the anterior neural folds (head fold : stage HH6 in the chick and 3 in the turtle).