Only three types of robust sink/source distributions at the BT surface are feasible irrespective to their phylogenetic origin. The evolution of gastrulation begins with the blastopore closure and continues through the formation of intercalary developmental stages that precede this closure leading to an increase in the blastopore diameter ( retrograde evolution). The BT domain is the region of maximum difference in the planar surface flow velocity and, as a consequence, of maximum variability of the gastrulation movements. The blastopore arises as a singular point of the vector field movement of a spherical surface having a zero velocity of the planar flow being surrounded by a toroidal surface (“border torus,” BT). ![]() ![]() ![]() The blastopore evolution is considered as a model example of the evolution of morphogenesis mechanism based on dipole interactions between the sources and sinks of epithelial sheet surface energy.
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