In organisms that reproduce sexually, once a sperm fertilizes an egg cell, the result is a cell called the zygote that has all the DNA of two parents.
In plants, animals, and some protists, the zygote will begin to divide by mitosis to produce a multicellular organism.
In animals, the development of the zygote into an embryo proceeds through specific recognizable stages of blastula, gastrula, and organogenesis.
The blastula stage typically features a fluid-filled cavity, the blastocoel, surrounded by a sphere or sheet of cells, also called blastomeres.
During gastrulation the cells of the blastula undergo coordinated processes of cell division, invasion, and/or migration to form two (diploblastic) or three (triploblastic) tissue layers.
In triploblastic organisms, the three germ layers are called endoderm, ectoderm and mesoderm.
However, the position and arrangement of the germ layers are highly species-specific, depending on the type of embryo produced.
In vertebrates, a special population of embryonic cells called the neural crest has been proposed as a "fourth germ layer," and is thought to have been an important novelty in the evolution of head structures.