File:Inner cell mass cartoon.jpg

From Embryology

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The Three Germ Layers of Embryonic Development

During the third week of human embryonic development, the following layers form and will eventually differentiate and form specific adult tissues (some examples are shown in the cartoon and below).

Germ Layers:

  1. Ectoderm - epidermis of skin (outer layer of skin) and nervous system.
  2. Mesoderm - connective tissues (bone, muscle, cartilage, blood)
  3. Endoderm - lining of the gut, lungs, endocrine organs.


  • These same three layers will form the same adult tissues in all mammalian, and more widely vertebrate (animals with a backbone), embryos.
    • The term "germ layers" refers to the fact that cell layers will form all adult tissues.
    • The term should not be confused with "germ cells", which refers to the cells that will form either eggs or spermatozoa.
    • The term "derm" means skin as in layer.
  • The layers form by a process called gastrulation.
    • This is therefore called the gastrula stage.
    • These are temporary layers and do not exist as layers in either the later embryo or adult.
  • Gastrulation will occur at different times in different species embryos.
    • In humans, this occurs about 3 weeks after fertilisation and also after implantation in the uterus.

Links: Gastrulation

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