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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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