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Updated: May 7, 2025
The former owes its color to a green pigment diffused chiefly through the ectoderm, but has comparatively few algae in its endoderm; while in the latter the pigment is present in much smaller quantity; but the endoderm cells are crowded by algae.
Afterwards there are slight histological differentiations in its ectoderm, though the entoderm remains a single stratum of cells. We find the first differentiation of epithelial and stinging cells, or of muscular and neural cells, in the thick ectoderm of the hydra.
The body consists of two layers of cells, the ectoderm on the outside and the entoderm lining the digestive cavity. Between these two is a structureless, elastic membrane, which tends to keep the body moderately expanded.
Thus that greater impressibility and contractility, which in the rudest animal types characterize the units of the ectoderm, characterize also the units of the primitive social stratum which controls and fights; since impressibility and contractility are the respective roots of intelligence and strength.
This is shown in the fact that each entodermal cell digests its own particles of food, although the nutriment once digested diffuses to all parts of the body. Also almost any part of the animal containing both ectoderm and entoderm can be cut off and will develop into a new animal.
Between the ectoderm and entoderm of the gastrula, in the space occupied by the supporting membrane of hydra, a new layer of cells, the mesoderm, appears. This has been produced by the rapid growth and reproduction of certain cells of the entoderm which have migrated, so to speak, into this new position.
To begin with the differentiation into ectoderm and endoderm, the organism is comparable to a human community made up of military and agricultural classes.
Their vegetative functions are accomplished by the simple visceral layer, and their animal functions by the simple cutaneous layer. In these cases the simple cell-layer of the ectoderm is at once skin, locomotive apparatus, and nervous system. Epidermic cells of a human embryo of two months.
Huxley, in describing the development of the Hydrozoa the endoderm and ectoderm. This primary division marks out a fundamental contrast of parts in the future organism. From the mucous layer, or endoderm, is developed the apparatus of nutrition; while from the serous layer, or ectoderm, is developed the apparatus of external action.
In higher forms it becomes of continually greater importance, until finally nearly all the organs of the body develop from it. In our bodies only the lining of the mid-intestine and of its glands has arisen from the entoderm. And only the epidermis, or outer layer of our skin, and the nervous system and parts of our sense-organs have arisen from the ectoderm.
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