from The American Heritage® Dictionary of the English Language, 4th Edition
- n. The outermost of the three primary germ layers of an embryo, from which the epidermis, nervous tissue, and, in vertebrates, sense organs develop.
- n. The outer layer of a diploblastic animal, such as a jellyfish.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. Outermost of the three tissue layers in the embryo of a metazoan animal. Through development, it will produce the epidermis (skin) and nervous system of the adult.
from the GNU version of the Collaborative International Dictionary of English
- n. The outer layer of the blastoderm; epiblast.
- n. The external skin or outer layer of an animal or plant, this being formed in an animal from the epiblast. See Illust. of blastoderm.
from The Century Dictionary and Cyclopedia
- n. The completed outer layer of cells, or outer blastodermic membrane, in all metazoan animals, formed by the cells of the epiblast, and primitively constituting the outer wall of the whole body, as the endoderm does that of the body-cavity; an epiblast, ectoblast, or external blastoderm.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
- n. the outer germ layer that develops into skin and nervous tissue
Sorry, no etymologies found.
Both the skin and the brain are derived from the same embryonic tissue, which is known as the ectoderm.
The outer layer of this ‘gastrula’ is called the ectoderm, the inner layer is the endoderm, and there are also some cells thrown into the space between the ectoderm and endoderm, which are called mesoderm.
Thus a portion of the upper marginal zone of the blastula or early gastrula, or else a piece of the roof of the archenteron of the mature gastrula was planted in the blastocoele of a young gastrula and so brought beneath the ectoderm from the beginning; it was demonstrated that these portions were able to induce neural plate.
The terms ectoderm and entoderm were introduced by Allman  in 1853 for the two cell-layers in the Hydrozoa.
A cell mass is first produced, and the cells in this differentiate into three layers called ectoderm, entoderm and mesoderm, from which the external and internal surfaces and the enclosed tissues respectively develop, and the different organs are produced by growth of the cells of certain areas of these layers.
The point to remember is, that in all cases a gastrula is an open sac composed of two layers of cells -- the outer layer being called the ectoderm, and the inner the endoderm.
At the very early stages, the cells are lumped into 3 layers, called the ectoderm, mesoderm, and endoderm.
The cord is covered by a layer of ectoderm which is continuous with that of the amnion, and its various constitutents are enveloped by embryonic gelatinous tissue, jelly of Wharton.
The outer layer, from which come the external skin and the muscles, was then called by Allman (1853) the "ectoderm" (outer layer, or skin); the inner layer, which forms the alimentary and reproductory organs, was called the "entoderm" (= inner layer).
They are adaptable animals, eating small crustaceans that are found in abundant numbers in lakes and in rivers and switching to molecular dietary absorption through their ectoderm when crustacean populations are scarce.