With rise in evolution, and during the progress of human embryonic development, these bones become fewer through their early gristly union or their synostosis. The openings in the skull resultant on the deficiencies in the chondrocranium are larger in the sauropsida (birds and reptiles) than in the ichthyopsida (amphibious and fish); in the monotremata (egg-laying mammals) than in the sauropsida; in the marsupials (pouched mammals) than in the monotremata, and in the higher mammals than in the marsupials. The development of the brain therefore depends on the growing and expanding power of the secondary skull formed by the dermal bones. These, considered as bones, are degenerate from the high type of the vertebræ, and are a mere reminiscence of that outer skeleton whereby early fish and reptiles emulated the lobster. The influence of any check to development such as produces degeneracy, is exerted first on the development of the bone itself, and finally on the relation to other bones by dovetailing. In accordance with the general laws governing growth, deficiency in one place is apt to result in increase elsewhere. The brain-protective function of the dermal bones being later in development than their old armour function, is apt to be checked by degeneracy in two ways; in the first the bone does not grow in size or sufficiently to unite with its fellows, or this growth occurs only for the benefit of the bone itself, through Spencer's law of individuation, so that union with the other bones occurs too early for the benefit of the organism as a whole. To the factors underlying this is due the non-increase in intellect after puberty which occurs in the higher apes, and in some of the lower races of men. These checks also tend to the nutritional benefit of the older primary skull, whence result the irregularities in development that constitute so many of the stigmata of the degenerate cranium. The sutures sometimes do not form because sufficient gristle is not produced to fill the gaps. (Fig. 8.) These secondary gaps are often filled by new dermal bones called Wormian. Sometimes this deficiency coexists with too early synostosis elsewhere. [Illustration: FIG. 8.] Degenerate skulls have therefore been divided from the standpoint of these various unions (by synostosis or otherwise) and non-unions of the sutures, on the principle that premature synostosis of a suture produces shortness of the diameter, perpendicular to the direction of the obliterated suture.[189] The bones stop growing prematurely at the seat of the synostosis, but the unaffected borders continue growing. The following types result on this principle: [Illustration: FIG. 9.]