Yolk plug in gastrula is future

The correct option is B Posterior end
In animals like Echinoderms, the embryo is converted into a double-walled, cup-shaped structure. The new internal wall lines the archenteron, the newly formed cavity of the developing gut. The opening of the archenteron to the exterior, the blastopore, is the site of the future anus in deuterostomes.This simple type of gastrulation cannot occur in the amphibian embryo, because the large yolk-laden cells in the vegetal half of the blastula obstruct any inward movement at the vegetal pole. Instead, cells from the animal pole move down the embryo surface; when they reach the region derived from the gray crescent, they move into the interior. This inward movement is accomplished as the cells change shape, first becoming flask- or bottle shaped (so that most of their mass is actually below the surface) and then sinking into the interior as they lose their remaining connections with other cells on the surface. This spot on the embryos surface, referred to as the dorsal lip of the blastopore, is marked by a dimple, shaped like a C lying on its side. As the process continues, the blastopore becomes ring-shaped as cells lateral, and then ventral, to its dorsal lip become involved in similar movements. The yolk-filled cells fill the space enclosed by the lips of the blastopore, forming the yolk plug. The archenteron forms and is lined on all sides by cells that have moved in from the surface. At first, the archenteron is a narrow slit, but it gradually expands at the anterior end of the embryo. As a result, the blastocoel progressively shrinks and eventually disappears. The yolk plug in gastrula is the future posterior end.