120 THE ORIGIN OF VERTEBRATES 
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 sut 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 phyloge- 
netic origin: as we pass from the higher vertebrates to the lower 
these structures can be traced back to a cartilaginous and mem- 
branous 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 
