It is probably in most cases derived from the endoderm, but the exact mode of its derivation is still somewhat obscure.
The outer is the Ectoderm and the inner is the endoderm or Hypoblast.
The terminal portion of the endoderm is solid, and contains calcareous concretions.
The endoderm contains in addition gland cells and nervous elements.
Between the ectoderm and endoderm a gelatinous supporting layer, termed the mesogloea, makes its appearance.
It has been shown that these asulcar filaments are derived from the ectoderm, the remainder from the endoderm.
In the course of development, however, cells from the ectoderm and endoderm may migrate into it.
The endoderm has cylindrical cells, each one of which has a flagellate hair.
Nettle-cells are occasionally found in the endoderm, but apparently do not originate in this layer.
It is enough for us that they always end in the formation of the two primary layers of ectoderm and endoderm.
endoderm en·do·derm (ěn'də-dûrm') or en·to·derm (ěn'tə-)
The innermost of the three primary germ layers of an embryo, developing into the gastrointestinal tract, the lungs, and associated structures. Also called hypoblast.
The innermost of the primary germ layers of an animal embryo. In vertebrates, the endoderm gives rise to the respiratory tract, gastrointestinal tract (except mouth and anus), glands associated with the gastrointestinal tract, bladder, and urethra. Compare ectoderm, mesoderm.