Stages of development in vertebrates.

Sperm penetrates an egg, the egg and sperm nuclei fuse, and a zygote forms. Mitotic cell divisions yield a ball of cells, a blastula. Each cell gets a different bit of the egg cytoplasm. Cell rearrangements and migrations form a gastrula, an early embryo that has primary tissue layers. Organs form as the result of tissue interactions that cause cells to move, change shape, and commit suicide. Organs grow in size, take on mature form, and gradually assume specialized functions. Fertilization Cleavage Gastrulation Organ Formation Growth, Tissue Specialization