The primary skull, as already stated, is practically an extension of the vertebræ, which send side outgrowths to cover the brain as the backbone covers the spinal cord. This primary skull extended in front of the notochord (the spinal cord of the human embryo, and the permanent spinal cord of the lancelet), where it gave off two trabeculæ cranii or front skull plates. Behind, the primary skull or chondrocranium gives off two occipal or rear skull plates. It gives off also two plates midway between the trabeculæ and occipitals, which, as they gradually enclose the primitive hearing apparatus, the otocysts (permanent in fish, and embryonic in man), are called periotic capsules. This primary skull is at first cartilaginous, as in sharks. With the increase in the size of the brain in evolution and in human embryology, this cartilaginous primary skull became insufficient to roof over the brain, and thus resulted gaps in it. The fontanelles, or soft places at the top, sides, and back of the head of the new-born child, are the remains of this failure of the chondrocranium, or primary skull, to cover the gains of the nervous system in the struggle for existence. This deficiency, resultant on advance in evolution, would have been a long-standing serious block to further advance, were it not that the skin of the mammal retained a function inherited from the reptiles and bony fish. These cavities were filled by dermal bones, which, at first serving merely as armour in the skin of the head, came to be protectors of the nervous system. The following bones represent these dermal bones in the embryonic human skull: The frontals, which form the chief part of the forehead; the sutures, or dovetails, of these normally disappear in the adult, so that the forehead seems to be but one bone. This union may not occur (Fig. 7), as in the case of the philosopher Kant, who had a frontal suture all his life. The dovetails are replaced by solid bone, through a process called synostosis. In the case of the frontal bone it is normal, and in the line of advance. Elsewhere in the skull it is often an expression of defect which may give rise to various cranial states which are either absolutely degenerate in type or degenerate only when occurring in certain races. The parietals and interparietals are also dermal bones which are united by synostosis to form the parietals or side bones of the normal adult skull. The nasal bones which, together with the vomer, form the nose, are likewise dermal bones, and so are the pterygoids and palatines. The maxillaries and præmaxillaries, which, with the mandibles, form the jaws, are dermal bones. The mandibles, however, are in part derived from the chondrocranium. [Illustration: FIG. 7.]