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Ch. 47—Animal Development Fertilization Fertilization Fertilization Fertilization Once fertilization is complete Animal Development • Cleavage—rapid mitotic cell division into smaller cells—blastomeres • Solid ball of cells— morula • Blastula produced when a cavity called blastocoel forms • Gastrulation— rearrangement of cells into 3 germ layers blastomeres Morula (solid ball of cells) Blastocyst • Blastula stage of mammalian embryos • Trophoblast layer enclosing the blastocoel – Chorion (outer embryonic layer) Blastocyst develops into gastrula Gastrulation • Cells rearrange and fold inward at the blastopore to form a gastrula with three-germ layers: – Ectoderm (skin, nails, teeth) – Mesoderm (skeletal, muscular, excretory, circulatory, reproductive systems; blood, bone, and muscle) – Endoderm (epithelial lining of digestive, respiratory, excretory tracts; liver, pancreas) Organogenesis • Development of 3 germ layers into the beginnings of organs – Notochord—stiff dorsal skeletal rod (from mesoderm) – Neural plate—will become brain and spinal cord (from ectoderm) – Somites—blocks of mesoderm arranged along notochord; sign of segmentation Amniotes • • • • Amnion—fluid filled cavity; protects embryo Allantois—disposal sac Chorion—exchange gases Yolk sac—provides nutrients Eutherian mammals