Blastula means the hollow sphere of cells called blastomeres and it has an inner fluid-filled cavity called a blastocoele. Blastoderm surrounds the blastocoele, and it consists of a layer of blastomeres. Blastopore occurs in gastrula and open into archentron. The blastopore is a pit in the side of the embryo, through which cells fated to be endodermal flow so that they leave the outer surface of the embryo and can create a new inner surface.