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To begin with the differentiation into ectoderm and endoderm, the organism is comparable to a human community made up of military and agricultural classes.
For our present knowledge of the power of intracellular digestion possessed by the endoderm cells of the lower invertebrates removes all difficulties both as to the mode of entrance of the algae, and its fate when dead. In short, we have here the relation of the animal and the vegetable world reduced to the simplest and closest conceivable form.
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.
Haeckel had already compared the yellow cells of Radiolarians to the so-called liver-cells of Velella; but the brothers Hertwig first recalled attention to the subject in 1879 by expressing their opinion that the well-known "pigment bodies" which occur in the endoderm cells of the tentacles of many sea-anemones were also parasitic algae.
Every one knows that all the colorless cells of a plant share the starch formed by the green cells; and it seems impossible to doubt that the endoderm cell or the Radiolarian, which actually incloses the vegetable cell, must similarly profit by its labors.
We saw that in primitive tribes, as in the simplest animals, there remains much community of function between the parts which are nominally different that, for instance, the class of chiefs long remains industrially the same as the inferior class; just as in a Hydra, the property of contractility is possessed by the units of the endoderm as well as by those of the ectoderm.
When there arises a decided differentiation between the governing and the governed, some amount of transfer begins between those inferior individuals who, as workers, come directly in contact with the products of the earth, and those superior ones who exercise the higher functions a transfer parallel to that which accompanies the differentiation of the ectoderm from the endoderm.
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.
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