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Updated: May 27, 2025
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.
In certain animals belonging to every one of the chief groups into which the Metazoa are divisible, the cells of the cell-aggregate which results from the process of yelk-division, and which is termed a morula, diverge from one another in such a manner as to give rise to a central space, around which they dispose themselves as a coat or envelope; and thus the morula becomes a vesicle filled with fluid, the planula.
The Norwegian Magosphaera planula, swimming about by means of the lashes or cilia at its surface. Section of Magosphaera planula, showing how the pear-shaped cells in the centre of the gelatinous ball are connected by a fibrous process.
From the moneron he proceeds to the amoeba a simple cell, with a kernel, which still corresponds to the egg of man in its first state. To the fourth stage he assigns the planæa, corresponding to the embryonic development of an albumen and the planula or ciliated larva.
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