part_of part of part of develops from simple squamous epithelium integument periderm In mice, the first non-basal layer formed at ~E9.5; it is a temporary structure composed of simple squamous epithelium that serves as the first barrier to the embryo's physical environment, exists throughout the entire keratinocyte stratification process, and sheds off at ~E17, when it is replaced by corneocytes[MP] Originally the epidermis is one layer thick, in most vertebrates it soon becomes a two-layered structure. The outer layer gives rise to the periderm. The periderm goes through distinct developmental phases and is ultimately sloughed into the amniotic fluid when differentiation of the underlying epidermal layers is complete. The function of the periderm is not known, but is thought to be related to transport/exchange between the fetus and the amniotic fluid (http://courses.washington.edu/hubio567/devbio/periderm.html) In some mammals, Eyelid Fusion is thought to be driven by a population of cells which are derived from the periderm, the outermost layer of the developing epidermis enveloping layer of ectoderm ecto-epithelium