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Related to ectoderm: mesoderm


layer of cells that covers the surface of an animal embryo after the process of gastrulation has occurred. This outer layer, together with the endodermendoderm
, in biology, inner layer of tissue formed in the gastrula stage of the developing embryo. At the end of the blastula stage, cells of the embryo are arranged in the form of a hollow ball.
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, or inner layer, is present in all early embryos. In the development of animals of the phyla PoriferaPorifera
[Lat.,=pore bearer], animal phylum consisting of the organisms commonly called sponges. It is the only phylum of the animal subkingdom Parazoa and represents the least evolutionarily advanced group of the animal kingdom.
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, CtenophoraCtenophora
, a small phylum of exclusively marine, invertebrate animals, commonly known as comb jellies. Because they are so delicate that specimens are difficult to collect, little was known about them until the advent of blue-water scuba and submersible collecting.
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, and CnidariaCnidaria
or Coelenterata
, phylum of invertebrate animals comprising the sea anemones, corals, jellyfish, and hydroids. Cnidarians are radially symmetrical (see symmetry, biological).
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, these two primary layers give rise to all the tissues and organs of the animals, a process known as diploblastic development. In higher animals, such as those of the phyla EchinodermataEchinodermata
[Gr.,=spiny skin], phylum of exclusively marine bottom-dwelling invertebrates having external skeletons of calcareous plates just beneath the skin. The plates may be solidly fused together, as in sea urchins, loosely articulated to facilitate movement, as in sea
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 and ChordataChordata
, phylum of animals having a notochord, or dorsal stiffening rod, as the chief internal skeletal support at some stage of their development. Most chordates are vertebrates (animals with backbones), but the phylum also includes some small marine invertebrate animals.
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, a third, middle layer, the mesodermmesoderm,
in biology, middle layer of tissue formed in the gastrula stage of the developing embryo. At the end of the blastula stage, cells of the embryo are arranged in the form of a hollow ball.
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, is formed between the ectoderm and endoderm during gastrulation, and the process is termed triploblastic development. In most embryos, differentiation of ectodermal tissue gives rise to epidermis and its specialized structures (scales, feathers, nails, and hair); some exocrine glands (sweat and sebaceous glands); some endocrine glands (the pineal body and the pituitary gland); the nervous system; and the organs of special sense (ear and eye). In animals of some phyla, such as the MolluscaMollusca
, taxonomic name for the one of the largest phyla of invertebrate animals (Arthropoda is the largest) comprising more than 50,000 living mollusk species and about 35,000 fossil species dating back to the Cambrian period.
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 and AnnelidaAnnelida
[Lat., anellus=a ring], phylum of soft-bodied, bilaterally symmetrical (see symmetry, biological), segmented animals, known as the segmented, or annelid, worms.
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, the fate of particular cells of the embryo is determined in the earliest stages of the fertilized egg and may even be fixed at or before fertilization.



(1) The outermost germ layer; the external layer of the embryo of multicellular animals in the gastrular stage. From the ectoderm are formed the integuments, nervous system, sensory organs, anterior and posterior sections of the digestive tract, external gills, and ectomesenchyme. In the Deuterostomia, all the elements derived from the ectoderm are formed as a result of the action upon it of the chordomesoderm, the entoderm, and their derivatives.

(2) The external wall of the body of coelenterates. The ectoderm consists of a single layer of cells—epithelial, epithelio-muscular, interstitial, and sensory cells, as well as stinging capsules.


The outer germ layer of an animal embryo. Also known as epiblast.
(invertebrate zoology)
The outer layer of a diploblastic animal.
References in periodicals archive ?
These studies elucidate the importance of the AER as well as non-ridge ectoderm for normal development of the limbs.
See figure) In placing the embryo outline over the iris chart, we see the areas of the ectoderm, the mesoderm, and the endoderm relate to the location of the various organs and body structures.
A lining of solenia-type gastroderm delimits the mesoglea from the gastric cavity, while a thin ectoderm (at the bottom of Fig.
in 1995 (87) analyzed the pharyngeal endoderm and ectoderm mandibular BMP-4 mutant mice, finding absence of the development of the early phases of the jaw, with implications in the almost complete loss of the jaw.
p63, a homologue of the cell-cycle regulator TP53 and plays a critically important role in regulation of the formation and differentiation of apical ectoderm ridge (AER).
The amniotic cavity develops from epiblast and three germ layers endoderm mesoderm and ectoderm are developed from hypoblst.
The most common is ectoderm in the form of skin hair and abundant sebum.
Experimental observations on the development of ectoderm free mesoderm of the limb bud in chick embryos.
After closure, the neuro-ectoderm separates from the cutaneous ectoderm (dysjunction), and the space between is filled by mesoderm.
PNETs are small round cell tumours that arise from cells of the primitive ectoderm and comprise 1% of all sarcomas.
The limb bud originates from the migration of the mesoderm onto the overlying ectoderm on the ventrolateral aspect of the embryo.