File:Inner cell mass cartoon.jpg

The Three Germ Layers of Embryonic Development

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

1. Ectoderm
2. Mesoderm
3. Endoderm

Note:

• These same three layers will form the same adult tissues in all mammalian, and more widely vertebrate, embryos.
• The layers form by a process called gastrulation (and this is therefore the gastrula stage).
• Gastrulation will occur at different times in different species embryos.
  • In humans, this occurs about 3 weeks after fertilisation and after implantation in the uterus.