You Pull an Unlabeled Video Out of a Box Labeled

You pull an unlabeled video out of a box labeled "Amphibian and sea urchin gastrulation." While watching the video, you decide it must be a video of a sea urchin embryo and not an amphibian embryo because you see that

A) the blastopore becomes the anus.
B) the blastopore becomes the mouth.
C) cells at the vegetal pole of the embryo invaginate.
D) cells on the side of the embryo involute into the interior of the embryo.
E) the very yolky cells at the vegetal pole are some of the last cells to move into the interior of the embryo.