The development of an animal embryo involves the following stages:
Fertilization: The fusion of an egg with a sperm. The fertilized egg is called a zygote.
Cleavage: The rapid mitotic cell divisions that immediately follow fertilization. The big zygote is divided into small cells called blastomeres. As the number of blastomeres increases to about 16 - 64, the early embryo is called a morula.
Blastulation: The formation of a blastula from a morula. The morula is an embryo filled evenly with blastomeres, but the blastula contains a fluid cavity called blastocoel. In mammals, the blastula is called a blastocyst.
Gastrulation: The formation of three germ layers (ectoderm, mesoderm, and endoderm) from the blastula or blastocyst.
Organogenesis: The formation of various organs. In humans, the ectoderm develops into skin cells and neurons; the mesoderm develops into muscle cells and red blood cells; the endoderm becomes lung and pancreatic cells (see the figure above).