The video sequence below provides an overview of the embryo's surface during gastrulation in Xenopus. The sequence can be divided into several steps, which can be seen more easily as you play back single frames of the movie: (1) The dorsal lip of the blastopore forms as a "dirty frown", which results from the concentration of pigment into the blastoporal pigment line as a consequence of the contraction of bottle cells. The blastoporal pigment line then spreads laterally and ultimately ventrally to complete the formation of blastopore. (2) The dorsal involuting marginal zone, which moves into the interior via involution, and non-involuting marginal zone, which remains on the surface of the embryo, then undergo convergence and extension to asymmetrically close the blastopore. (3) The movements of gastrulation conclude with the complete closure of the blastopore and the elongation of the embryo along the forming anterior-posterior axis.