The Early Developmental Changes in the Reptilian Ovum

Abstract

ABSTRACT Professors Kuppfer and Benecke have recently published a very interesting note on this subject. Their observations were made on Lacerta agilis and on Emgs Europea, in both of which types they found the embryonic changes to be closely alike. Segmentation takes place exactly as in birds, and the resulting blastoderm, which is thickened at its edge, spreads rapidly over the yolk. A small embryonic shield makes its appearance in the centre of the blastoderm shortly before the yolk is half enclosed. It is somewhat pyriform, and mainly distinguished from the remainder of the blastoderm by the more columnar character of its constituent epiblast cells. At the narrower end of the shield, which may be spoken of as posterior in relation to the future embryo, an invagination takes place, and gives rise to a sack, the blind end of which is directed forwards. The opening of the sack may be spoken of as the blastopore. A linear thickening of the epiblast arises in front of the blastopore, along which the medullary groove soon appears. In the cephalic region the medullary groove flattens out, and posteriorly the two medullary folds diverge, and enclose between them the blastopore, behind which they again meet. On the completion of the medullary canal the blastopore becomes obliterated ; but there can be but little doubt, although the authors of the paper do not appear to touch on this point, that on the closure of the medullary folds the medullary canal remains in direct communicatian with the cavity of the invaginated sack, so that the latter has exactly the same relation to the medullary canal as the embryonic structure, which I have called “the postanal section of the alimentary canal “in Elasmobranchii.