A blastula is a sphere of cells surrounding a blastocoel. The trophoblasts secrete fluid into the blastocoel. A seal is created by tight junctions of the epithelial cells that line the blastocoel. These presumptive PGCs are brought to the endoderm of the blastocoel by gastrulation. This buckles inwards towards the blastocoel in a process called invagination. The blastula stage of early embryo development begins with the appearance of the blastocoel. The endoderm of the archenteron will fuse with the ectoderm of the blastocoel wall. This layer surrounds the inner cell mass and a fluid-filled cavity known as the blastocoel. As cleavage continues, the cavity expands to become the developed blastocoel. These cells then dissociate and ingress into the blastocoel and are called the primary mesenchyme. The avian blastocoel is important during the development of the primitive streak. Only when the blastocoel is formed does the early embryo become a blastula. The blastocoel, trophoblast cells, and inner cell mass cells are hallmarks of the blastocyst. As cells migrate into the blastocoel, they undergo an epithelial-to-mesenchymal transformation. The blastocoel is a crucial component of amphibian embryo development.