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Development

  • The first phase of embryological development is cleavage. During this process, the embryo undergoes a series of cell divisions but little overall growth.
  • During the next stage, a hollow ball of cells called a blastula forms. A fluid-filled blastocoel is within the blastula.
  • Cells then relocate and differentiate to form a gastrula. The gastrula consists of three germ layers: the ectoderm, mesoderm and endoderm. The following organs and systems develop from each layer:
  • Ectoderm - epidermis, hair, nails, nervous system, cornea and lens of the eye, pituitary glands.
  • Mesoderm – circulatory system, musculoskeletal system, excretory system and most of the reproductive system.
  • Endoderm – linings of the GI and respiratory tracts; pancreas, liver and gall bladder; thymus; thyroid gland
  • Organogenesis is the formation of organs. During this process, the cells differentiate. The notochord and neural tube form early in chordate development.
  • Induction refers to the influence of one group of cells on the development of another group of cells. This process is mediated by diffusible factors.

Development

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AP Biology Intro