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Embryonic Development Gametes (eggs/sperm) are produced by what process? Meiosis Egg + Sperm = Zygote ---> Embryo (multicellular) The zygote (one fertilized cell) becomes trillions of highly specialized cells by what process? Mitosis Zygote Embryo What happens to the size of individual cells as the zygote undergoes Mitosis to produce a multicellular embryo? Two main stages of Embryonic Development: 1. Cleavage - Zygote undergoes rapid cell division (Mitosis) to produce a multicellular hallow ball of cells called a blastula (about 100 cells). Cleavage helps to organize the embryo into developmental regions. o The position of each new cell in the embryo promotes expression of particular groups of genes Chemical signals in the cytoplasm activate specific genes and directs development. o A particular cell only expresses genes that code for proteins with a function in that cell or that need to be produced by that cell. Only some genes activated in each cell. Embryo Implantation 2. Gastrulation – During this stage of development the embryo accumulates more cells and the cells sort into three layers. The embryo is now called the Gastrula. As the embryo develops, individual cells undergo Cellular Differentiation – becomes specialized in their structure and function. Three Embryonic Tissue Layers: o Ectoderm – Forms from the outer layer of the gastrula. Develops into the outer skin of the organism and its nervous system o Endoderm – Forms from the embryonic digestive tract. Develops into the inner lining of the digestive tract and lungs o Mesoderm – Forms from the space between the ectoderm and endoderm. Develops into most other organs Differentiation Flow Chart Egg + Sperm Zygote Blastula Gastrula Gastrulation Fertilization Cleavage – rapid mitotic divisions. Embryo – 9 weeks * 50 days after fertilization * Ball of 100 cells * Fluid filled * Outer layer – called trophoblast * Inner cells called Inner cell mass (Stem cells) * Formation of 3 cell layers: 1. Ectoderm – outer layer (skin and central nervous system) 2. Endoderm – Digestive tract and lungs. 3. Mesoderm – most other organs 60 days