Embryology Early Embryonic Development CLEAVAGE • After fertilization, cleavage (cell division) occurs as the cells divide but the embryo does not become larger; the cells become smaller with each division. Early Embryonic Development • Identical twins form if the zygote splits into two identical cells. Identical twins come from the same egg and sperm and always are the same sex. • Fraternal twins form if two different eggs were fertilized from two different sperms at the same time so they can be different sexes. MORULA The resulting mass of cells is called a morula- a round ball of cells. BLASTULA As cell division continues, the mass of cells spread out called a blastula. GASTRULA Next, cells migrate inward producing a hollow embryo with an layer to the outside called a gastrula, much like a partially deflated basket ball. Below: Various stages of embryonic development in a sea star (Echinodermata) Three layers of cells form called germ layers. •Ectoderm (outer layer) differentiates into the brain and nervous system. •Endoderm (inner layer) forms the digestive tract. •Mesoderm (middle layer) differentiates into the muscles, skeleton, circulatory &reproductive organs. NEURULA Next the embryo develops a spinal cord and brain from a long, thickened fold in the ectoderm. The embryo at this stage of development is called a neurula. Vocab Zygote Cleavage Blastula Ectoderm Endoderm Gastrulation Fraternal Twins Identical Twins Mesoderm Morula Neurula Essential Questions • What are the stages of embryonic development immediately after the egg is fertilized? • What do the first three layers of our cells differentiate into? • How are identical and fraternal twins formed?.