ectoderm

The terms ectoderm and entoderm were introduced by Allman in 1853 for the two cell-layers in the Hydrozoa.

Ontogeny teaches unmistakably that its elementary organs, the epidermic cells, develop entirely from the ectoderm.

Amongst other structures which arise from the important layer of ectoderm are the teeth.

The skin corresponds with the ectoderm of Hydra, although it is a vastly more complicated affair.

In the typical invertebrate eye, on the contrary, the retinal cells are differentiated from the external ectoderm.