{120}Is there any evidence which makes it possible to conceive the method by which the vertebrate skeleton may have arisen from the skeletal tissues of an arthropod? By the vertebrate skeleton I mean the bony and cartilaginous structures which form the backbone, the cranio-facial skeleton, the pectoral and pelvic girdles, and the bones of the limbs. I do not include the notochord in these skeletal tissues, because there is not the slightest evidence that the notochord played any part in the formation of these structures; the notochordal tissue is something _sui generis_, and never gives rise to cartilage or bone. The notochord happens to lie in the middle line of the body and is very conspicuous in the lowest vertebrate; with the development of the backbone the notochord becomes obliterated more and more, until at last it is visible in the higher vertebrates only in the embryo; but that obliteration is the result of the encroachment of the growing bone-masses, not the cause of their growth. Although, then, the notochord may in a sense be spoken of as the original supporting axial rod of the vertebrate, it is so different to the rest of the endo-skeleton, has so little to do with it, that the consideration of its origin is a thing apart, and must be treated by itself without reference to the origin of the cartilaginous and bony skeleton. THE COMMENCEMENT OF THE BONY SKELETON IN THE VERTEBRATE. What is the teaching of the vertebrate? What evidence is there as to the origin of the bony skeleton in the vertebrate phylum itself? The axial bony skeleton of the higher Mammalia consists of two parts, (1) the vertebral column with its attached bony parts, and (2) the cranio-facial skeleton. Of these two parts, the bony tissue of the first arises in the embryo from cartilage, of the second partly from cartilage, partly from membrane. In strict accordance with their embryonic origin is their phylogenetic origin: as we pass from the higher vertebrates to the lower these structures can be traced back to a cartilaginous and membranous condition, so that, as Parker has shown, the cranio-facial bony skeleton of the higher vertebrates can be derived directly from a non-bony cartilaginous skeleton, such as is seen in Petromyzon and the cartilaginous fishes. Balfour, in his "Comparative Embryology," states that the {121}primitive cartilaginous cranium is always composed of the following parts:-- 1. A pair of cartilaginous plates on each side of the cephalic section of the notochord known as the parachordals (_pa.ch._, Fig. 49; _iv._, Fig. 48). These plates, together with the notochord (_ch._) enclosed between them, form a floor for the hind and mid-brain. [Illustration: FIG. 48.--EMBRYO PIG, TWO-THIRDS OF AN INCH LONG (from PARKER), ELEMENTS OF SKULL SEEN FROM BELOW.