noun. The innermost of the three primary germ layers of an animal embryo, developing into the gastrointestinal tract, the lungs, and associated structures.

noun. In zoology, the completed inner layer of cells in all metazoan animals, formed by the cells of the hypoblast or endoblast, and representing, under whatever modification, the lining of the enteron: opposed to ectoderm.

noun. The inner layer of the skin or integument of an animal.

noun. The innermost layer of the blastoderm and the structures derived from it; the hypoblast; the entoblast. See Illust. of ectoderm.

noun. One of the three tissue layers in the embryo of a metazoan animal. Through development, it will produce the digestive system of the adult.

noun. the inner germ layer that develops into the lining of the digestive and respiratory systems

"This stage of differentiation consists in the formation of either a pouch or an additional layer between the ectoderm and the endoderm, which is called the mesoderm."

ectoderm  mesoderm  secondary endoderm  

germ layer  

bolus  collapsibility  constrictor  ectoderm  endothelium  

ectoderm