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Updated: June 6, 2025
Then every mesoblast contracts; the contraction deepens, till it is divided across in both directions, separating thus into four parts, then into eight, then into sixteen, and so on, till every cell is crowded with hundreds of minute mesoblasts, each containing the indication of a central dot or entoblast.
We have further to note that in the embryos of Metazoa at all advanced in organization, there arises between these two layers a third the mesoblast. The origin of this is seen in types where the developmental process is not obscured by the presence of a large food-yolk.
Minor qualifications being admitted, there remain the broad general facts, that out of that part of the external layer which remains permanently external, are developed all the structures which carry on intercourse with the medium and its contents, active and passive; out of the introverted part of this external layer, are developed the structures which carry on intercourse with the quasi-external substances that are taken into the interior solid food, water, and air; while out of the mesoblast are developed structures which have never had, from first to last, any intercourse with the environment.
In all the higher animals a layer of cells makes its appearance between the hypoblast and the epiblast, and is termed the mesoblast.
The optic cup, or its contained fluid, is one chemical compound; epithelium is another; mesoblast is a third.
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