Long used by developmental biologists and biology students, the relatively large and transparent eggs of the Amphibia allow convenient study of vertebrate embryology. The fertilized egg or oocyte is said to have an animal pole where nuclear material concentrates and a vegetal pole where the yolk is primarily located. The zygote divides by cleavage forming a morula (16-64 cell stage) and is followed by blastulation, the process of forming a blastocoel (64-128 cells or blastula stage), the prominent cavity in the animal hemisphere above the yolk which leads to gastrulation, the formation of the three layers of the larval and adult frog: endoderm, ectoderm, and mesoderm. Gastrulation establishes the multi-layered body plan of organization for amphibians and higher vertebrates.