He shewed, by these methods, that, though both skull and vertebral column are segmented, the one and the other, after an early stage, are fashioned on lines so different as to exclude the possibility of regarding the details of each as mere modifications of a common type.  “The spinal column and the skull start from the same primitive condition, whence they immediately begin to diverge.”  “It may be true to say that there is a primitive identity of structure between the spinal or vertebral column and the skull; but it is no more true that the adult skull is a modified vertebral column than it would be to affirm that the vertebral column is a modified skull.”  Taking the embryological facts, he shewed that the skull arose out of elements quite different from those of the vertebral column.  The notochord alone is common to both.  The skull is built up of longitudinal cartilaginous pieces, now known as the “parachordals” and “trabeculae,” of sense capsules enclosing the nose and ear, and of various roofing bones.  In the historical development of the skull three grades become apparent; a primitive stage, as seen in Amphioxus, where there is nothing but a fibrous investment of the nervous structures; a cartilaginous grade, as seen in the skate or shark, where the skull is formed of cartilage, very imperfectly hardened by earthy deposits; a bony stage, seen in most of the higher animals.  He shewed that in actual development of the higher animals these historical grades are repeated, the skull being at first a mere membranous or fibrous investment of the developing nervous masses, then becoming cartilaginous, and, lastly, bony.  He made some important prophetic remarks as to the probable importance that future embryological work would give to the distinction between cartilage and membrane bones—­a prophecy that has been more than fully realised by the investigations of Hertwig and of others.  Our present knowledge of the skull differs from Huxley’s conception practically only in a fuller knowledge of details.  We know now that throughout the series there is a primitive set of structures common to all animals higher in the scale than Amphioxus, and forming the base and lateral walls of the skull.  This is termed the Chondrocraninm, because it is laid down in cartilage; it is composed of the separate elements which Huxley indicated, and, in different animals, as Huxley suggested, the exact limits of the ossification of the primitive cartilages differ in extent, but occur in homologous situations.  This primitive skull is roofed over by a series of membrane bones which have no connection in origin with the other portions of the skull, and which have no representative in the vertebral column, but which are the direct descendants of the bony scales clothing the external skin in cartilaginous fishes.  In one respect only was Huxley erroneous.  Partly by inadvertence, and partly because the minute details of vertebrate embryology became really familiar to zooelogists only after the elaborate work of Balfour of Cambridge, Huxley, in his account of the formation of the first beginnings of the skeleton in the embryo, made confusion between the walls of the primitive groove, which, in reality, give rise to the nervous structures, and those embryonic tissues which form the skeletal system.