Formation of Blastopore
The slit such as opening of the initial groove is the opening of the developing blastopore and its dorsal rim is termed as the dorsal lip of blastopore. As gastrulation carries on the two ends of the groove extend on either side horizontally in a wide area with the border among the lateral marginal and vegetal zones. Finally they meet at the mid-line of the future ventral side of the embryo so completing the blastopore with the creation of its lateral and ventral lips. The mass of yolky endodermal cells of vegetal zone, now encircled through the four blastopore lips, protrudes by the blastopore and is called the yolk plug. Later, the lateral lips of blastopore grow towards each other over the yolky plug totally enclosing the yolky endoderm as the blastopore is reduced to a narrow vertical slit by the end of gastrulation.