THE DEVELOPMENT OP VERTEBRATA 



tadpole at about the time of appearance of the hind legs.) 

 Gradually a matrix is secreted by these cells, and the tube 

 takes on more and more the appearance of cartilage. An 

 extension dorsalwards from the tube now takes place ; on 

 either side of the spinal cord a paired metameric series of 

 cartilages is formed, alternating with the mesoblastic somites. 

 The undivided tube around the notochord will give rise to 

 the centra of the vertebra ; the metameric cartilages, to the 

 neural arches. The tube around the notochord presently 

 becomes thicker, at intervals which correspond to the 

 somites in frog and chick, but alternate with them in the 

 rabbit. In this way the notochord becomes constricted at 

 metameric intervals, and eventually cut up into separate 

 masses. Next, the cartilaginous tube becomes meta- 

 merically divided by transverse rifts into a series of carti- 

 laginous centra which (like the neural arches) alternate 

 with the somites. Consequently the isolated remnants of 

 the notochord persist for a time within the centra of frog 

 and chick, but "between the centra of the rabbit (as in the 

 dogfish). In the frog the hinder portion of the tube is 

 never metamerically segmented, but forms the urostyle. 



Later still, ossification occurs (chap, vi., 11), and the 

 cartilaginous vertebrae are replaced by vertebrae of carti- 

 lage-bone epiphyses being formed only in the case of the 

 rabbit. In this process, too, the final remnants of the 

 notochord are absorbed, except that in the rabbit a little 

 persists in the middle of the fibro-cartilaginous inter- 

 vertebral discs which are unossified parts of the original 

 tube. 



The development of the skull has been fully treated of in 

 chapter xiv., and of the rest of the skeleton little need be said. It 

 is worth noting, however, that the cartilages of the branchial arches 

 arise (like those of the larynx and trachea), in splanchnic meso- 

 blast ; whereas the limb-girdles and limb-skeletons are formed in 

 somatic mesoblast. This fact effectually disposes of a theory, ably 

 advocated at one time, that the limb-girdles were homologous with 

 gill-arches. 



8. The Nervous System. The general facts of the 

 development of the brain and spinal cord from the primitive 

 condition of a cellular tube have been indicated in chapter ix. 

 ZOOL. 18 



