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Under this is a soft medullary substance, which consists of entodermic cells with vacuoles. These gonads are not yet independent sexual glands, but sexually differentiated cell-groups in the medullary substance, or, in other words, parts of the gut-wall. Most of the Platodaria have not the muscular pharynx, which is very advanced in the Turbellaria and Trematoda.
At the lower or vegetal pole of the ovum a crown of eight large entodermic cells remains for a long time unchanged, while the other cells divide, owing to the formation of a series of horizontal circles, into an increasing number of crowns of sixteen cells each. This layer is the blastoderm, the simple epithelium from the cells of which all the tissues of the body proceed.
In consequence of these important folding-processes in the primitive gut, the simple entodermic tube divides into four different sections: 1. underneath, at the ventral side, the permanent alimentary canal or permanent gut; 2. above, at the dorsal side, the axial rod or chorda; and 3. the two coelom-sacs, which immediately sub-divide into two structures:
At first the chorda only consists of a single row of large entodermic cells. It is afterwards composed of several rows of cells. In the Ascidia-larva, also, the chorda develops from the dorsal middle part of the primitive gut, while the two coelom-pouches detach themselves from it on both sides. The simple body-cavity is formed by the coalescence of the two.
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