English: Gastrulation at 16° and 26°C. This movie shows gastrulation at 16°C (left) and 26°C (right) starting at the late blastula stage, ending after blastopore closure, as the neural plate converges on the dorsal side (top). Embryos are in a vegetal pole view. Movies were rotated so that the dorsal side is towards is at the top of the image, cropped to fit, and contrast was adjusted to optimize the image. Note that the embryo at 16°C was filmed at 2 minutes per frame, while the embryo at 26°C was filmed at 1 minute per frame. For this combined movie their playback rates were adjusted to match, and they were synchronized to the beginning of involution on the dorsal side. Early on in each embryo, the apices of bottle cells contract strongly. This begins on the dorsal side of the embryo, eventually forming a dark ring of cells with narrowed apices around the blastopore. Soon after the ring fully forms, the superficial layer of tissue outside the blastopore on the dorsal side begins to involute (“dorsal involution”), appearing to roll inwards over the blastopore lip and inside the embryo. Involution progresses around the blastopore, until it occurs on the ventral side as well (“ventral involution”). This time difference is tp. The blastopore finally closes (“blastopore closure”) sometime after involution begins on the ventral side, but involution continues after blastopore closure. Note that the ectoderm and mesoderm move vegetally while the ring of bottle cells contracts, before superficial involution begins. However, we defined the time for blastopore closure (tc) based on the beginning of involution because it is a much more clearly marked time point.