When an animal embryo starts to form, it develops a round, hollow grouping of cells called a blastula. These cells then begin to differentiate into distinct layers known as germ layers, which will eventually develop into different groups of organs and body parts. Most animals develop from blastulas that have three germ layers: an outermost layer called the ectoderm, a middle section known as the mesoderm, and an inner layer called the endoderm. These animals are known as triploblastic. Some, more primitive, animals, such as jellyfish, have blastulas with only two layers, the ectoderm and the endoderm; these are diploblastic.