noun[mass noun] Zoology & Embryology
The outermost layer of cells or tissue of an embryo in early development, or the parts derived from this, which include the epidermis, nerve tissue, and nephridia. Compare with endoderm and mesoderm.
- The presumptive neural tube tissues consist of a layer of ectoderm along the dorsal midline of the embryo, between the notochord and an outer layer of epidermis.
- The branchial arches begin as cylindrical cores of mesenchyme sandwiched between continuous sheets of epidermal ectoderm and internal endoderm.
- The anthozoans are diploblastic animals developing from two epithelial germ cell layers: the ectoderm, facing the seawater, and the endoderm, facing the gastrovascular cavity or coelenteron.
- Example sentences
- In most polychaetes, the ectodermal and mesodermal derivatives of the growth zone become incorporated into segmental tissues in an anterior-posterior sequence.
- For example, chaetal sacks are one of the few ectodermal features of polychaetes that regularly express engrailed, and chaetae are the primary indurated elements in the ectoderm.
- At the very tip of the limb bud is a thickening in the ectoderm - the apical ectodermal ridge or apical ridge, which runs along the dorso-ventral boundary.
Mid 19th century: from ecto- 'outside' + Greek derma 'skin'.
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