Endoderm is one of three principal germinal layers of cells that are formed in early in embryonic development. Endoderm is also referred to as entoderm.

The other principal germinal layers (e.g., layers of cells from which body tissues develop) are ectoderm and mesoderm. Endoderm comprises the innermost germinal layer from which the epithelium of the digestive system and most of the respiratory system are derived. Endodermal cells also develop into parts of the urethra and bladder.

Membranes derived from endoderm are in contact with ectoderm derived membranes at the mouth and anus.

In the embryonic disk ectoderm and endoderm border a third intermediate mesodermal layer. When the embryonic disk ultimately folds into a tube the basic "tube within a tube" plan of development becomes evident. A core endodermal tube establishes a primitive digestive pathway bounded by an oral orifice and an anal orifice. Around that innermost tube...