The part played by the ectoderm and endoderm appeared quite clear, and the former might reasonably be regarded as the cutaneous investment of primitive multicellular animals, while the latter might be regarded as their organ of digestion.
Although the central nervous system is almost always developed from the ectoderm of the embryo, the same cannot be said of the peripheral nerve trunks.
Thus the ectoderm gives rise to the epidermis, to the nervous system, and to the lining of the stomodaeum and proctodaeum, if such parts of the alimentary canal are present.
As a general rule the ectoderm and endoderm, which may be called the primary layers, come first, and later the mesoderm is developed from one or other of them.
In it cleavage gives rise to a solid mass, which divides by delamination into two layers, the ectoderm and endoderm.
These thickened tracts of ectoderm in Peripatus and a few other forms can be clearly seen to surround the blastopore.
In this order the axis is formed as an ingrowth of the ectoderm of the base of the mother zooid of the colony, the cavity of the ingrowth being filled by a horny substance secreted by the ectoderm.
The body-wall is highly muscular and, except in a few probably specialized cases, possesses chitinous spines, the setae, which are secreted by the ectoderm and are embedded in pits of the skin.
In the course of development, however, cells from the ectoderm and endoderm may migrate into it.
The ectoderm beneath each fold becomes detached from the surface of the basal plate, and both it and the mesogloea are folded conformably with the endoderm.
Most commonly the spicule-forming cells pass out of the ectoderm and are imbedded in the mesogloea, where they may remain separate from one another or may be fused together to form a strong mass.
The body cavity, which is a space between the ectoderm and alimentary canal, is not lined by mesoderm and is traversed by a few muscular fibres.
At the anterior end of the preoral lobe is a nervous thickening of the ectoderm called the apical plate (1).
This stage of development, called gastrula, is similar to a cup with a double wall, of which the outer is the ectoderm and the inner the endoderm.
One end of the blastopore becomes nearly closed, and an ingrowth of ectoderm takes place around it to form the stomodaeum or fore-gut and mouth.
The rectal peduncle or pedicle of invagination; its attachment to the ectoderm is coincident with the hindmost extremity of the elongated blastopore of fig.
The undifferentiated skin-layer or ectoderm of the Gastraea is the simple stratum of cells from which the differentiated sense-organs of all the Metazoa (including the Vertebrates) have been evolved.
In these cases the simple cell-layer of the ectoderm is at once skin, locomotive apparatus, and nervous system.
There is, however, an invagination of the ectoderm at the mouth, which has given rise to a muscular pharynx (sd).
The leaf-shaped bilateral-symmetrical body has only one gut-opening, and develops the first trace of a nervous centre from the ectoderm in the middle line of the back (Figures 2.
The cell-pit of the ectoderm that lies underneath is rather thick, and represents the first rudiment of a neural ganglion (vertical brain or acroganglion).
We find the first differentiation of epithelial and stinging cells, or of muscular and neural cells, in the thick ectoderm of the hydra.
A remarkable peculiarity of the Cyclostomes lies in the fact that the pituitary ingrowth of ectoderm does not, as in other forms, become involved in the inpushing of ectoderm which forms the buccal cavity.
The rudiment of the central nervous system has the form of a solid keel-like ingrowth of ectoderm along the mid-dorsal line, which only secondarily becomes hollowed out--just as happens in Teleostean fishes.
You may note that in some Hydrae there is a swelling or bulging of theectoderm of the body-wall in the region just below the tentacles.
Many of the endoderm as well as ectoderm cells have muscle-processes which spread out from the base of the cell and which serve to contract and expand the body.
Some of the interstitial cells become modified and pushed up between the ectoderm cells to form cnidoblast cells.
Note, in a cross-section, that there are small ovoid or cuboid cells at the bases of the large ectoderm cells.
Wheeler, the amnion is ruptured and turned back from covering the germ band, enclosing the yolk dorsally and becoming finally absorbed, as the ectoderm of the germ band itself spreads to form the dorsal wall.
Between the ectoderm cells and the entoderm cells which line the gastrula cavity there arises a hyaline structureless layer, which is more closely attached to the ectoderm than to the entoderm, and is probably derived from the former.
It is effected by protoplasmic processes of the outer ring of ectoderm cells, which, together with the otherectoderm cells, now become amoeboid.
After the formation of the structureless layer between the ectoderm and entoderm, calcareous spicules make their appearance in it as delicate unbranched rods pointed at both extremities.
The two layers of the gastrula may now be spoken of as ectoderm and entoderm.
The figure shews the amoeboid ectoderm cells (ec) derived from the granular cells of the earlier stage, and the columnar entoderm cells, lining the gastrula cavity, derived from the ciliated cells of the earlier stage.
Between the Ectoderm and the Endoderm a layer of cells called the Mesoderm or Mesoblast is next formed, and from these three layers all the parts of the embryo are built up.
The layer of the blastoderm, between the ectoderm and endoderm; mesoblast.
Defn: A thin gelatinous tissue separating the ectoderm and endoderm in certain coelenterates.
A central cavity is commonly developed, and the cells around it are at first arranged in two layers, -- the ectoderm and endoderm.
The outer is the Ectoderm and the inner is the Endoderm or Hypoblast.
Ec, Ectoderm or skin layer; En, Endoderm or stomach layer; m, mouth leading into the enteric cavity.
The skin corresponds with the ectoderm of Hydra, although it is a vastly more complicated affair.
Between the Ectodermand Endoderm lies an intermediate layer the Mesoderm.
According to Haeckel, the origin of the generative products in the mesoderm is a heterotopic phenomenon, for he considers that they must have originated phylogenetically in one of the two primary layers, ectoderm or endoderm.
The cerebral ganglion was formed by the ends of the nerve-cord growing up round the oesophagus and fusing with the paired "sense-plates" which develop from the ectoderm of the head.
Coelentera were homologous with the ectoderm and endoderm of the germ was thus fully confirmed and greatly extended.
The class out of which governmental organization originates, is, as we have said, analogous in its relations to the ectoderm of the lowest animals and of embryonic forms.
Later on the same anal edge of the ectoderm forms another cellular layer, the endoderm proper, which forms a continuous sheet below the ectoderm.
The mesoderm also originates at the anal side of the ectoderm and extends in two bands right and left between ectoderm and endoderm.
From one part of the periphery of the ectoderm proliferation of cells takes place and gives rise to a layer of scattered nuclei over the whole surface of the yolk.
Later, in front of the primitive streak, a thickened band of ectoderm appears, broadening out posteriorly.
Then the ectoderm on the surface over the cup begins to thicken, grows into the cup itself, and ultimately forms a rounded hollow mass which we afterwards recognise as the lens of the eye.
And so we see that from this wonderful layer of ectoderm there comes gradually into existence not only the brain itself and the spinal cord, with all the nerves, but also the special sense organs of sight and smell.
Amongst other structures which arise from the important layer of ectoderm are the teeth.
From the ectodermarise the following structures:-- Epidermis or skin.
Evidently then this ectoderm or outer layer is of the very greatest importance in embryology, since from it arise all those parts of the embryo itself which are the most important in its future life.
But perhaps the most remarkable of all is the list of structures which take their origin from the ectoderm of the embryo (see A).
The wall of the embryonic vesicle now consists throughout of two layers of cells, the ectoderm without and the entoderm within.
Generally (if not always) the cells of the skin-layer or ectoderm (Figures 1.
The outer envelope of dark smaller cells forms the ectodermor skin-layer.
The outer stratum is the skin-layer, orectoderm (Figures 1.
In consequence of this, the animal cells of the ectoderm are darker than the vegetal cells of the entoderm.
At the same time, the entoderm spreads further and further over the inner surface of the ectoderm (e).
Around it the ectoderm is much thicker, and forms the border of the primitive mouth, the most important part of the embryo (Figure 1.
The germinative area or gastric disk of the animal consists at first (like the germinal disk of birds and reptiles) merely of the two primary germinal layers, the ectoderm and entoderm.
Into these fins, which are largely cuticular and strengthened by radiating bars, a single layer of ectoderm cells projects.
A thin gelatinous tissue separating the ectoderm and endoderm in certain c\'d2lenterates.
As development proceeds, this central nervous system sinks inwards, leaving as its connection with the ectoderm the sensory nerves of the skin.
The external cells of the ectoderm draw in their vibrating, ciliary hairs, whereas, on the contrary, the inner cells of the entoderm begin to form them.
The outer single layer of cells which constitutes the surface of the vesicle is the ectoderm or epiblast.
The test is really a ciliated velum developed in the normal position at the apical pole but reflected backwards in such a way as to cover the original ectoderm except at the posterior end.
The above list will hopefully give you a few useful examples demonstrating the appropriate usage of "ectoderm" in a variety of sentences. We hope that you will now be able to make sentences using this word.
