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But our mid-intestine is still the greatly elongated archenteron of the gastrula. We may therefore compare the hydra or gastrula to a little portion of the lining of the human mid-intestine covered with a little flake of epidermis. This much the hydra has attained.
The space occupied by the yolk corresponds to the archenteron or primitive digestive cavity; and the opening at the end to the primitive mouth or blastopore. Ectoderm and entoderm unite around the mouth. Both the blastosphere and gastrula often swim freely by flagella.
The wall of the planula is next pushed in on one side, or invaginated, whereby it is converted into a double-walled sac with an opening, the blastopore, which leads into the cavity lined by the inner wall. This cavity is the primitive alimentary cavity or archenteron; the inner or invaginated layer is the hypoblast; the outer the epiblast; and the embryo, in this stage, is termed a gastrula.
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