 During the process of gastrulation, a special type of cells called bottle cells invaginates a hole on the surface of the blastula which is called the dorsal lip of the blastopore.Once this lip has been established, the bottle cells will extend inward and migrate along the inner wall of the blastula known as the roof of the blastocoel.The once superficial cells of the animal pole are destined to become the cells of the middle germ layer called the mesoderm.