The endoderm is the innermost of the three embryonic germ layers.  It lies beneath the ectoderm and mesoderm.  This germ layer gives rise to:

- the epithelial lining of the respiratory tract
- the gastrointestinal tract
- accessory organs of the digestive tract

Endoderm

The mesoderm is the middle of the three embryonic germ layers.  It lies in between the endoderm and the ectoderm.  This germ layer gives rise to:

- muscles
- skeleton
- connective tissue
- the urogenital system
- blood
- blood vessels
- part of the heart

Mesoderm

The ectoderm is either the outside (in amphibians) or dorsal (in birds and mammals) surface of the embryo after gastrulation.  This germ layer gives rise to:

- neural tube
- neural crest
- epidermis

Ectoderm

The germ layers are the three layers of the embryo that are formed in the gastrulation process.  They give rise to all of the tissues of the body except for germ cells.  The layers include:

- endoderm
- mesoderm
- ectoderm

Michigan State University

Gastrulation occurs in embryogenesis after cleavage.  The blastomeres that make up the blastula extensively rearrange themselves to contain three germ layers: the endoderm, the mesoderm, and the ectoderm.  These three layers will give rise to all of the organs of the body.  After gastrulation, the embryo is said to be a gastrula.  

Gastrulation

Barresi, M. J., &amp; Gilbert, S. F. (2020). Developmental Biology (12th ed.). New York: Sinauer.

Developmental Biology (12th ed.)

Germ Layers

- anterior-posterior axis
- dorsal-ventral axis
- right-left axis

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