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 mi 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 



