During cleavage, the cytoplasm of the zygote is divided into many different cells called blastomeres. By the end of cleavage, the embryo is called a blastula, which is a ball of cells surrounding a fluid-filled cavity called a blastocoel.
During gastrulation, the embryo is called a gastrula. In this stage, cells undergo dramatic rearrangement, forming the embryonic germ layers and other structures, such as the blastopore. Once gastrulation is complete, the major body axes of the embryo (anterior-posterior and dorsal-ventral) are visible.
During organogenesis, the three embryonic germ layers develop into the rudiments of organs. Structures such as the notochord, the neural tube, and somites form. Somite cells undergo differentiation, giving rise to different body tissues and organs, including the skeleton, muscles, and skin.