germ layer

In embryology, a layer of cells that can be distinguished during the development of a fertilized egg. Most animals have three such layers: the inner, middle, and outer. These differentiate to form the various body tissues.

The inner layer (endoderm) gives rise to the gut, the middle one (mesoderm) develops into most of the other organs, while the outer one (ectoderm) gives rise to the skin and nervous system. Simple animals, such as sponges, lack a mesoderm.