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I. The chondrocranium, jaws, arches, and muscles of a partly grown larva.

Owing to the struggle for existence which occurs at puberty between the old type of the chondrocranium and its new type as supplemented by the dermal bones, the nervous system takes a distorted ply which arrests both the bodily, nervous, and mental development at certain points.

The deficiencies in the chondrocranium appear in the occipital bone, which requires a small portion of the dermal bone to complete it on each side.

These two conditions result from the inability of the dermal bones to fill at the proper period the gaps in the chondrocranium.

Behind, the primary skull or chondrocranium gives off two occipal or rear skull plates.