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Updated: July 31, 2024
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.
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.
But once or twice lately there had been signs that a third interest was creeping into his life, and he had found his attention wandering from the fate of the mesoblastic somites or the probable meaning of the blastopore, to the thought of the girl with the brown eyes who sat at the table before him. She was a paying student; she descended inconceivable social altitudes to speak to him.
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