The theory of the skull which held the field was known as the vertebral theory. The great bulk of the nervous system of vertebrate animals consists of a mass of tissue lying along the dorsal line of the body and enclosed in a cartilaginous or bony sheath. The nerve tissue is the brain and spinal cord; the sheath is the skull in front and the vertebral column along the greater part of the length of the animal. The brain may be taken simply as an anterior portion of the nerve mass, corresponding in a general way to an expansion of the spinal cord in the region of the anterior limbs and an expansion in the region of the hind limbs, the latter indeed having recently been shown in some extinct creatures to surpass the brain in size. In a similar simple fashion the skull may be taken as an expanded anterior part of the vertebral column, serving as an expanded box for the brain, just as in the regions of the pectoral and pelvic expansions of the cord there are similar expansions of the surrounding bony case. We know now, from greater knowledge of its embryological development, that the brain contains structures quite peculiar to itself, and differs from the spinal cord in kind as well as in size; but, at the same time, when the vertebral theory of the skull was inaugurated, embryological knowledge and the importance of its relation to anatomical structure were less considered. What Huxley did was to show that the skull, in its mode of origin and real nature, was not merely an expanded portion of the vertebral column, but that it differed from it in kind.