1895.] On the Vertebral Column of Amphibia, $-c. 257 



chlorine at atmospheric pressure. The terminals were then con- 

 nected to the electrodes of a battery giving a potential difference of 

 1,200 volts, but not the slightest movement of the drop of acid could 

 be detected. 



I wish to acknowledge the help I have received in making the 

 preceding experiment from my assistant, Mr. E. Everett. 



X. "On the Evolution of the Vertebral Column of Amphibia 

 and Amniota." By HANS GADOW, M.A., Ph.D., F.R.S. 

 Received June 20, 1895. 



(Abstract.) 



The key to the solution of the composition of the vertebral column 

 is given by the metameric repetition of the four pairs of symmetri- 

 cally arranged cartilaginous elements, the origin of which we have 

 traced in Fishes, namely 



One pair of basidorsalia. 



One pair of basiventralia (with its lateral outgrowths = ribs or 

 pleurapophyses, and haemal, ventral outgrowths = haemal 

 arches, chevrons, wedge-bones, heemapophyses) . 



One pair of interdorsalia. 



One pair of interventralia. 



The first of these four pairs is always present and forms the neural 

 arch. Of the other three pairs any one may be suppressed, some- 

 times even two in the same skleromere. 



The vertebrae of the Amphibia and Amniota possess only arch- 

 centra, because the chordal sheath takes no share in their formation. 



According to the composition of these arco-genous centra or bodies 

 we distinguish the following lines of evolution : 



I. Earliest primitive condition. All the -four pairs of arcualia are 



present : 



1. Either as separate pieces, of which the interventralia are 



the smallest, e.g., Archegosaurus, Chelydosaurus, Larvce of 

 Urodela. 



2. Or the basidorsalia and basiventralia fuse together and 



form the middle of the vertebral body, while the inter- 

 basalia form neutral zones of intervertebral cartilage. 

 Such PSEUDOCENTRA are those in the tail of Urodela. 



II. The interventralia are reduced. 



1. The other three pairs remain separate, and the inter- 

 dorsalia tend to form the centrum. (NOTOCENTRODS 



